tree, c++: Consider TARGET_EXPR invariant like SAVE_EXPR [PR118509]

My October PR117259 fix to get_member_function_from_ptrfunc to use a
TARGET_EXPR rather than SAVE_EXPR unfortunately caused some regressions as
well as the following testcase shows.
What happens is that
get_member_function_from_ptrfunc -> build_base_path calls save_expr,
so since the PR117259 change in mnay cases it will call save_expr on
a TARGET_EXPR.  And, for some strange reason a TARGET_EXPR is not considered
an invariant, so we get a SAVE_EXPR wrapped around the TARGET_EXPR.
That SAVE_EXPR <TARGET_EXPR <...>> gets initially added only to the second
operand of ?:, so at that point it would still work fine during expansion.
But unfortunately an expression with that subexpression is handed to the
caller also through *instance_ptrptr = instance_ptr; and gets evaluated
once again when computing the first argument to the method.
So, essentially, we end up with
(TARGET_EXPR <D.2907, ...>, (... ? ... SAVE_EXPR <TARGET_EXPR <D.2907, ...>
 ... : ...)) (... SAVE_EXPR <TARGET_EXPR <D.2907, ...> ..., ...);
and while D.2907 is initialized during gimplification in the code dominating
everything that uses it, the extra temporary created for the SAVE_EXPR
is initialized only conditionally (if the ?: condition is true) but then
used unconditionally, so we get
pmf-4.C: In function ‘void foo(C, B*)’:
pmf-4.C:12:11: warning: ‘<anonymous>’ may be used uninitialized [-Wmaybe-uninitialized]
   12 |   (y->*x) ();
      |   ~~~~~~~~^~
pmf-4.C:12:11: note: ‘<anonymous>’ was declared here
   12 |   (y->*x) ();
      |   ~~~~~~~~^~
diagnostic and wrong-code issue too.

The following patch fixes it by considering a TARGET_EXPR invariant
for SAVE_EXPR purposes the same as SAVE_EXPR is.  Really creating another
temporary for it is just a waste of the IL.

Unfortunately I had to tweak the omp matching code to be able to accept
TARGET_EXPR the same as SAVE_EXPR.

2025-01-20  Jakub Jelinek  <jakub@redhat.com>

	PR c++/118509
gcc/
	* tree.cc (tree_invariant_p_1): Return true for TARGET_EXPR too.
gcc/c-family/
	* c-omp.cc (c_finish_omp_for): Handle TARGET_EXPR in first operand
	of COMPOUND_EXPR incr the same as SAVE_EXPR.
gcc/testsuite/
	* g++.dg/expr/pmf-4.C: New test.