AVR: PR118012 - Try to work around sick code from match.pd.

This patch tries to work around PR118012 which may use a
full fledged multiplication instead of a simple bit test.
This is because match.pd's

/* (zero_one == 0) ? y : z <op> y -> ((typeof(y))zero_one * z) <op> y */
/* (zero_one != 0) ? z <op> y : y -> ((typeof(y))zero_one * z) <op> y */

"optimizes" code with op in { plus, ior, xor } like

  if (a & 1)
    b = b <op> c;

to something like:

  x1 = EXTRACT_BIT0 (a);
  x2 = c MULT x1;
  b = b <op> x2;

or

  x1 = EXTRACT_BIT0 (a);
  x2 = ZERO_EXTEND (x1);
  x3 = NEG x2;
  x4 = a AND x3:
  b = b <op> x4;

which is very expensive and may even result in a libgcc call for
a 32-bit multiplication on devices that don't even have MUL.
Notice that EXTRACT_BIT0 is already more expensive (slower, more
code, more register pressure) than a bit-test + branch.

The patch:

o Adds some combiner patterns that try to map sick code back
  to a bit test + branch.

o Adjusts costs to make MULT (x AND 1) cheap, in the hope that the
  middle-end will use that alternative (which we map to sane code).

o On devices without MUL, 32-bit multiplication was performed by a
  library call, which bypasses the MULT (x AND 1) and similar patterns.
  Therefore, mulsi3 is also allowed for devices without MUL so that
  we get at MULT pattern that can be transformed.  (Though this is
  not possible on AVR_TINY since it passes arguments on the stack).

o Add a new command line option -mpr118012, so most of the patterns
  and cost computations can be switched off as they have
  avropt_pr118012 in their insn condition.

o Added sign-extract.0 patterns unconditionally (no avropt_pr118012).

Notice that this patch is just a work-around, it's not a fix of the
root cause, which are the patterns in match.pd that don't care about
the target and don't even care about costs.

The work-around is incomplete, and 3 of the new tests are still failing.
This is because there are situations where it does not work:

* The MULT is realized as a library call.

* The MULT is realized as an ASHIFT, and the ASHIFT again is transformed
  into something else.  For example, with -O2 -mmcu=atmega128,
  ASHIFT(3) is transformed into ASHIFT(1) + ASHIFT(2).

	PR tree-optimization/118012
	PR tree-optimization/118360
gcc/
	* config/avr/avr.opt (-mpr118012): New undocumented option.
	* config/avr/avr-protos.h (avr_out_sextr)
	(avr_emit_skip_pixop, avr_emit_skip_clear): New protos.
	* config/avr/avr.cc (avr_adjust_insn_length)
	[case ADJUST_LEN_SEXTR]: Handle case.
	(avr_rtx_costs_1) [NEG]: Costs for NEG (ZERO_EXTEND (ZERO_EXTRACT)).
	[MULT && avropt_pr118012]: Costs for MULT (x AND 1).
	(avr_out_sextr, avr_emit_skip_pixop, avr_emit_skip_clear): New
	functions.
	* config/avr/avr.md [avropt_pr118012]: Add combine patterns with
	that condition that try to work around PR118012.
	(adjust_len) <sextr>: Add insn attr value.
	(pixop): New code iterator.
	(mulsi3) [avropt_pr118012 && !AVR_TINY]: Allow these in insn condition.
gcc/testsuite/
	* gcc.target/avr/mmcu/pr118012-1.h: New file.
	* gcc.target/avr/mmcu/pr118012-1-o2-m128.c: New test.
	* gcc.target/avr/mmcu/pr118012-1-os-m128.c: New test.
	* gcc.target/avr/mmcu/pr118012-1-o2-m103.c: New test.
	* gcc.target/avr/mmcu/pr118012-1-os-m103.c: New test.
	* gcc.target/avr/mmcu/pr118012-1-o2-t40.c: New test.
	* gcc.target/avr/mmcu/pr118012-1-os-t40.c: New test.
	* gcc.target/avr/mmcu/pr118360-1.h: New file.
	* gcc.target/avr/mmcu/pr118360-1-o2-m128.c: New test.
	* gcc.target/avr/mmcu/pr118360-1-os-m128.c: New test.
	* gcc.target/avr/mmcu/pr118360-1-o2-m103.c: New test.
	* gcc.target/avr/mmcu/pr118360-1-os-m103.c: New test.
	* gcc.target/avr/mmcu/pr118360-1-o2-t40.c: New test.
	* gcc.target/avr/mmcu/pr118360-1-os-t40.c: New test.