Recognize MULT_HIGHPART_EXPR in tree-ssa-math-opts pass.

Recognize MULT_HIGHPART_EXPR in tree-ssa-math-opts pass.

This is the third iteration of a patch to perceive MULT_HIGHPART_EXPR
in the middle-end.  As they say "the third time's a charm".  The first
version implemented this in match.pd, which was considered too early.
https://gcc.gnu.org/pipermail/gcc-patches/2020-August/551316.html
The second version attempted to do this during RTL expansion, and was
considered to be too late in the middle-end.
https://gcc.gnu.org/pipermail/gcc-patches/2021-August/576922.html
https://gcc.gnu.org/pipermail/gcc-patches/2021-August/576923.html

This latest version incorporates Richard Biener's feedback/suggestion
to perceive MULT_HIGHPART_EXPR in one of the "instruction selection
passes", specifically tree-ssa-math-opts, where the recognition of
highpart multiplications takes place in the same pass as widening
multiplications.

With each rewrite, the patch is also getting more aggressive in the
set of widening multiplications that it recognizes as highpart multiplies.
Currently any widening multiplication followed by a right shift (either
signed or unsigned) by a bit count sufficient to eliminate the lowpart
is recognized.  The result of this shift doesn't need to be truncated.
As previously, this patch confirms the target provides a suitable
optab before introducing the MULT_HIGHPART_EXPR.  This is the reason
the testcase is restricted to x86_64, as this pass doesn't do anything
on some platforms, but x86_64 should be sufficient to confirm that the
pass is working/continues to work.

2022-01-11  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	* tree-ssa-math-opts.c (struct widen_mul_stats): Add a
	highpart_mults_inserted field.
	(convert_mult_to_highpart): New function to convert right shift
	of a widening multiply into a MULT_HIGHPART_EXPR.
	(math_opts_dom_walker::after_dom_children) [RSHIFT_EXPR]:
	Call new convert_mult_to_highpart function.
	(pass_optimize_widening_mul::execute): Add a statistics counter
	for tracking "highpart multiplications inserted" events.

gcc/testsuite/ChangeLog
	* gcc.target/i386/mult-highpart.c: New test case.