tree-ssa-math-opts: Small uaddc/usubc pattern matching improvement [PR79173]

tree-ssa-math-opts: Small uaddc/usubc pattern matching improvement [PR79173]

In the following testcase we fail to pattern recognize the least significant
.UADDC call.  The reason is that arg3 in that case is
  _3 = .ADD_OVERFLOW (...);
  _2 = __imag__ _3;
  _1 = _2 != 0;
  arg3 = (unsigned long) _1;
and while before the changes arg3 has a single use in some .ADD_OVERFLOW
later on, we add a .UADDC call next to it (and gsi_remove/gsi_replace only
what is strictly necessary and leave quite a few dead stmts around which
next DCE cleans up) and so it all of sudden isn't used just once, but twice
(.ADD_OVERFLOW and .UADDC) and so uaddc_cast fails.  While we could tweak
uaddc_cast and not require has_single_use in these uses, there is also
no vrp that would figure out that because __imag__ _3 is in [0, 1] range,
it can just use arg3 = __imag__ _3; and drop the comparison and cast.

We already search if either arg2 or arg3 is ultimately set from __imag__
of .{{ADD,SUB}_OVERFLOW,U{ADD,SUB}C} call, so the following patch just
remembers the lhs of __imag__ from that case and uses it later.

2023-06-20  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/79173
	* tree-ssa-math-opts.cc (match_uaddc_usubc): Remember lhs of
	IMAGPART_EXPR of arg2/arg3 and use that as arg3 if it has the right
	type.

	* g++.target/i386/pr79173-1.C: New test.