Improve ix86_expand_int_movcc to allow condition (mask) sharing.

Improve ix86_expand_int_movcc to allow condition (mask) sharing.

This patch modifies the way that ix86_expand_int_movcc generates RTL,
to allow the condition mask to be shared/reused between multiple
conditional move sequences.  Such redundancy is common when RTL
if-conversion transforms non-trivial basic blocks.

As a motivating example, consider the new test case:

int a, b, c, d;
int foo(int x)
{
    if (x == 0) {
        a = 3;
        b = 1;
        c = 4;
        d = 1;
    } else {
        a = 5;
        b = 9;
        c = 2;
        d = 7;
    }
    return x;
}

This is currently compiled, with -O2, to:

foo:	cmpl	$1, %edi
	movl	%edi, %eax
	sbbl	%edi, %edi
	andl	$-2, %edi
	addl	$5, %edi
	cmpl	$1, %eax
	sbbl	%esi, %esi
	movl	%edi, a(%rip)
	andl	$-8, %esi
	addl	$9, %esi
	cmpl	$1, %eax
	sbbl	%ecx, %ecx
	movl	%esi, b(%rip)
	andl	$2, %ecx
	addl	$2, %ecx
	cmpl	$1, %eax
	sbbl	%edx, %edx
	movl	%ecx, c(%rip)
	andl	$-6, %edx
	addl	$7, %edx
	movl	%edx, d(%rip)
	ret

Notice that the if-then-else blocks have been if-converted into four
conditional move sequences/assignments, each consisting of cmpl, sbbl,
andl and addl.  However, as the conditions are the same, the cmpl and
sbbl instructions used to generate the mask could be shared by CSE.

This patch enables that so that we now generate:

foo:    cmpl    $1, %edi
        movl    %edi, %eax
        sbbl    %edx, %edx
        movl    %edx, %edi
        movl    %edx, %esi
        movl    %edx, %ecx
        andl    $-6, %edx
        andl    $-2, %edi
        andl    $-8, %esi
        andl    $2, %ecx
        addl    $7, %edx
        addl    $5, %edi
        addl    $9, %esi
        addl    $2, %ecx
        movl    %edx, d(%rip)
        movl    %edi, a(%rip)
        movl    %esi, b(%rip)
        movl    %ecx, c(%rip)
        ret

Notice, the code now contains only a single cmpl and a single sbbl,
with result being shared (via movl).

2023-01-03  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.cc (ix86_expand_int_movcc): Rewrite
	RTL expansion to allow condition (mask) to be shared/reused,
	by avoiding overwriting pseudos and adding REG_EQUAL notes.

gcc/testsuite/ChangeLog
	* gcc.target/i386/cmov10.c: New test case.