i386: Prevent unwanted combine from LZCNT to BSR [PR101175]

i386: Prevent unwanted combine from LZCNT to BSR [PR101175]

The current RTX pattern for BSR allows combine pass to convert LZCNT insn
to BSR. Note that the LZCNT has a defined behavior to return the operand
size when operand is zero, where BSR has not.

Add a BSR specific setting of zero-flag to RTX pattern of BSR insn
in order to avoid matching unwanted combinations.

2021-06-23  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
	PR target/101175
	* config/i386/i386.md (bsr_rex64): Add zero-flag setting RTX.
	(bsr): Ditto.
	(*bsrhi): Remove.
	(clz<mode>2): Update RTX pattern for additions.

gcc/testsuite/

	PR target/101175
	* gcc.target/i386/pr101175.c: New test.