Souper says:

; Function: fn1
%0:i32 = var 
%1:i1 = eq 0:i32, %0
%2:i32 = zext %1
%3:i32 = udiv 1:i32, %2
%4:i1 = eq 0:i32, %3
cand %4 0:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_404/foo.bc

LLVM says:

define void @fn1() #0 {
entry:
  %0 = load i32* @b, align 4, !tbaa !1
  %cmp = icmp eq i32 %0, 0
  %conv = zext i1 %cmp to i32
  %div = udiv i32 1, %conv
  %tobool = icmp eq i32 %div, 0
  br i1 %tobool, label %lor.rhs, label %lor.end

lor.rhs:                                          ; preds = %entry
  store i32 0, i32* @a, align 4, !tbaa !1
  br label %lor.end

lor.end:                                          ; preds = %entry, %lor.rhs
  ret void
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_404/foo.c -o reduce_404/foo.bc

C source code:

int a, b;
void fn1() { 1 / (b == 0) || (a = 0); }

x86-64 from LLVM:

fn1:                                    # @fn1
	cmpl	$0, b(%rip)
	sete	%al
	movzbl	%al, %ecx
	movl	$1, %eax
	xorl	%edx, %edx
	divl	%ecx
	testl	%eax, %eax
	jne	.LBB0_2
	movl	$0, a(%rip)
.LBB0_2:                                # %lor.end
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_404/foo.c -S -o -

x86-64 from GCC:

fn1:
	movl	b(%rip), %eax
	testl	%eax, %eax
	je	.L1
	movl	$1, %eax
	xorl	%ecx, %ecx
	cltd
	idivl	%ecx
	testl	%eax, %eax
	je	.L4
.L1:
	rep ret
.L4:
	movl	$0, a(%rip)
	ret

COMMAND: gcc -w -O3 reduce_404/foo.c -S -o -