phiopt: Optimize x < 0 ? ~y : y to (x >> 31) ^ y [PR96928]

phiopt: Optimize x < 0 ? ~y : y to (x >> 31) ^ y [PR96928]

As requested in the PR, the one's complement abs can be done more
efficiently without cmov or branching.

Had to change the ifcvt-onecmpl-abs-1.c testcase, we no longer optimize
it in ifcvt, on x86_64 with -m32 we generate in the end the exact same
code, but with -m64:
        movl    %edi, %eax
-       notl    %eax
-       cmpl    %edi, %eax
-       cmovl   %edi, %eax
+       sarl    $31, %eax
+       xorl    %edi, %eax
        ret

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96928
	* tree-ssa-phiopt.c (xor_replacement): New function.
	(tree_ssa_phiopt_worker): Call it.

	* gcc.dg/tree-ssa/pr96928.c: New test.
	* gcc.target/i386/ifcvt-onecmpl-abs-1.c: Remove -fdump-rtl-ce1,
	instead of scanning rtl dump for ifcvt message check assembly
	for xor instruction.