Dead Code Elimination Regression when using __builtin_unreachable (trunk vs 15)

[thetheodor]

https://godbolt.org/z/71djY498P

Given the following code:

void foo(void);
static int a, c;
static int *b = &a, *d = &c, *g = &a;
static char e, h;
static short f;
int main() {
    f = 5 - c;
    if (f ^ 0)
        ;
    else {
        foo();
    }
    *b = c >= 0;
    h = *g;
    char j = *d = h;
    e = j << 2;
    int i = e;
    c = -3;
    if (!(((i) >= 4) && ((i) <= 4))) {
        __builtin_unreachable();
    }
}

clang-trunk -O3 does not eliminate the call to foo:

main:                                   # @main
# %bb.0:
	xorl	%eax, %eax
	cmpb	$0, c(%rip)
	movl	$65533, %ecx                    # imm = 0xFFFD
	cmovel	%eax, %ecx
	cmpl	$5, %ecx
	jne	.LBB0_2
# %bb.1:
	pushq	%rax
	callq	foo@PLT
	addq	$8, %rsp
.LBB0_2:
	movb	$1, c(%rip)
	xorl	%eax, %eax
	retq
.Lfunc_end0:

clang-15.0.7 -O3 eliminates the call to foo:

main:                                   # @main
# %bb.0:
	movb	$1, c(%rip)
	xorl	%eax, %eax
	retq
.Lfunc_end0:

Bisects to 0f32a5dea0e9ef5a52865f9fd285b394d46babaf (@rotateright )