i386: Introduce peephole2 to use flags from CMPXCHG more [PR96189]

i386: Introduce peephole2 to use flags from CMPXCHG more [PR96189]

CMPXCHG instruction sets ZF flag if the values in the destination operand
and EAX register are equal; otherwise the ZF flag is cleared and value
from destination operand is loaded to EAX. Following assembly:

        movl    %esi, %eax
        lock cmpxchgl   %edx, (%rdi)
        cmpl    %esi, %eax
        sete    %al

can be optimized by removing the unneeded comparison, since set ZF flag
signals that no update to EAX happened.

2020-15-07  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:
	PR target/95355
	* config/i386/sync.md
	(peephole2 to remove unneded compare after CMPXCHG): New pattern.

gcc/testsuite/ChangeLog:
	PR target/95355
	* gcc.target/i386/pr96189.c: New test.