dojump: Optimize a == a or a != a [PR98169]

dojump: Optimize a == a or a != a [PR98169]

If the backend doesn't have floating point EQ or NE comparison, dojump.c
splits it into ORDERED && UNEQ or UNORDERED || LTGT.  If both comparison
operands are the same, we know the result of the second comparison though,
a == b is equivalent to a ord b and a != b is equivalent to a unord b,
and thus can just use ORDERED or UNORDERED.

On the testcase, this changes f1:
-	ucomiss	%xmm0, %xmm0
-	movl	$1, %eax
-	jp	.L3
-	jne	.L3
-	ret
-	.p2align 4,,10
-	.p2align 3
-.L3:
 	xorl	%eax, %eax
+	ucomiss	%xmm0, %xmm0
+	setnp	%al
and f3:
-	ucomisd	%xmm0, %xmm0
-	movl	$1, %eax
-	jp	.L8
-	jne	.L8
-	ret
-	.p2align 4,,10
-	.p2align 3
-.L8:
 	xorl	%eax, %eax
+	ucomisd	%xmm0, %xmm0
+	setnp	%al
while keeping the same code for f2 and f4.

2020-12-10  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98169
	* dojump.c (do_compare_rtx_and_jump): Don't split self-EQ/NE
	comparisons, just use ORDERED or UNORDERED.

	* gcc.target/i386/pr98169.c: New test.