ix86: Add peephole2 for *add<mode>3_cc_overflow_1 followed by matching memory store...

ix86: Add peephole2 for *add<mode>3_cc_overflow_1 followed by matching memory store [PR94857]

The following peephole2 changes:
-	addl	(%rdi), %esi
+	xorl	%eax, %eax
+	addl	%esi, (%rdi)
 	setc	%al
-	movl	%esi, (%rdi)
-	movzbl	%al, %eax
 	ret
on the testcase.  *add<mode>3_cc_overflow_1, being an add{l,q} insn, is
commutative, so if TARGET_READ_MODIFY_WRITE we can replace
addl (%rdi), %esi; movl %esi, (%rdi)
with
addl %esi, (%rdi)
if %esi is dead after those two insns.

2020-05-08  Jakub Jelinek  <jakub@redhat.com>

	PR target/94857
	* config/i386/i386.md (peephole2 after *add<mode>3_cc_overflow_1): New
	define_peephole2.

	* gcc.target/i386/pr94857.c: New test.