loop-split improvements, part 2

this patch fixes profile update in the first case of loop splitting.
The pass still gives up on very basic testcases:

__attribute__ ((noinline,noipa))
void test1 (int n)
{
  if (n <= 0 || n > 100000)
    return;
  for (int i = 0; i <= n; i++)
    {
      if (i < n)
	do_something ();
      if (a[i])
	do_something2();
    }
}

Here I needed to do the conditoinal that enforces sane value range of n.
The reason is that it gives up on:
      !number_of_iterations_exit (loop1, exit1, &niter, false, true)
and without the conditonal we get assumption that n>=0 and not INT_MAX.
I think from overflow we shold derive that INT_MAX test is not needed and since
the loop does nothing for n<0 it is also just an paranoia.

I am not sure how to fix this though :(.  In general the pass does not really
need to compute iteration count.  It only needs to know what direction the IVs
go so it can detect tests that fires in first part of iteration space.

Rich, any idea what the correct test should be?

In testcase:
  for (int i = 0; i < 200; i++)
    if (i < 150)
      do_something ();
    else
      do_something2 ();
the old code did wrong update of the exit condition probabilities.
We know that first loop iterates 150 times and the second loop 50 times
and we get it by simply scaling loop body by the probability of inner test.

With the patch we now get:

  <bb 2> [count: 1000]:

  <bb 3> [count: 150000]:    <- loop 1 correctly iterates 149 times
  # i_10 = PHI <i_7(8), 0(2)>
  do_something ();
  i_7 = i_10 + 1;
  if (i_7 <= 149)
    goto <bb 8>; [99.33%]
  else
    goto <bb 17>; [0.67%]

  <bb 8> [count: 149000]:
  goto <bb 3>; [100.00%]

  <bb 16> [count: 1000]:
  # i_15 = PHI <i_18(17)>

  <bb 9> [count: 49975]:    <- loop 2 should iterate 50 times but
			       we are slightly wrong
  # i_3 = PHI <i_15(16), i_14(13)>
  do_something2 ();
  i_14 = i_3 + 1;
  if (i_14 != 200)
    goto <bb 13>; [98.00%]
  else
    goto <bb 7>; [2.00%]

  <bb 13> [count: 48975]:
  goto <bb 9>; [100.00%]

  <bb 17> [count: 1000]:   <- this test is always true becuase it is
			      reached form bb 3
  # i_18 = PHI <i_7(3)>
  if (i_18 != 200)
    goto <bb 16>; [99.95%]
  else
    goto <bb 7>; [0.05%]

  <bb 7> [count: 1000]:
  return;

The reason why we are slightly wrong is the condtion in bb17 that
is always true but the pass does not konw it.

Rich any idea how to do that?  I think connect_loops should work out
the cas where the loop exit conditon is never satisfied at the time
the splitted condition fails for first time.

Before patch on hmmer we get a lot of mismatches:
Profile report here claims:
dump id |static mismat|dynamic mismatch                                     |
        |in count     |in count                  |time                      |
lsplit  |      5    +5|   8151850567  +8151850567| 531506481006       +57.9%|
ldist   |      9    +4|  15345493501  +7193642934| 606848841056       +14.2%|
ifcvt   |     10    +1|  15487514871   +142021370| 689469797790       +13.6%|
vect    |     35   +25|  17558425961  +2070911090| 517375405715       -25.0%|
cunroll |     42    +7|  16898736178   -659689783| 452445796198        -4.9%|
loopdone|     33    -9|   2678017188 -14220718990| 330969127663             |
tracer  |     34    +1|   2678018710        +1522| 330613415364        +0.0%|
fre     |     33    -1|   2676980249     -1038461| 330465677073        -0.0%|
expand  |     28    -5|   2497468467   -179511782|--------------------------|

With patch

lsplit  |      0      |            0             | 328723360744        -2.3%|
ldist   |      0      |            0             | 396193562452       +20.6%|
ifcvt   |      1    +1|     71010686    +71010686| 478743508522       +20.8%|
vect    |     14   +13|    697518955   +626508269| 299398068323       -37.5%|
cunroll |     13    -1|    489349408   -208169547| 257777839725       -10.5%|
loopdone|     11    -2|    402558559    -86790849| 201010712702             |
tracer  |     13    +2|    402977200      +418641| 200651036623        +0.0%|
fre     |     13      |    402622146      -355054| 200344398654        -0.2%|
expand  |     11    -2|    333608636    -69013510|--------------------------|

So no mismatches for lsplit and ldist and also lsplit thinks it improves speed by
2.3% rather than regressig it by 57%.

Update is still not perfect since we do not work out that the second loop
never iterates.

Ifcft wrecks profile by desing since it insert conditonals with both arms 100%
that will be eliminated later after vect.  It is not clear to me what happens
in vect though.

Bootstrapped/regtested x86_64-linux, comitted.

gcc/ChangeLog:

	PR middle-end/106923
	* tree-ssa-loop-split.cc (connect_loops): Change probability
	of the test preconditioning second loop to very_likely.
	(fix_loop_bb_probability): Handle correctly case where
	on of the arms of the conditional is empty.
	(split_loop): Fold the test guarding first condition to
	see if it is constant true; Set correct entry block
	probabilities of the split loops; determine correct loop
	eixt probabilities.

gcc/testsuite/ChangeLog:

	PR middle-end/106293
	* gcc.dg/tree-prof/loop-split-1.c: New test.
	* gcc.dg/tree-prof/loop-split-2.c: New test.
	* gcc.dg/tree-prof/loop-split-3.c: New test.