Souper says:

; Function: fn1
%0:i32 = var 
%1:i32 = subnsw 1:i32, %0
%2:i1 = ne %0, %1
cand %2 1:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_341/foo.bc

LLVM says:

define i32 @fn1() #0 {
entry:
  %0 = load i32* @a, align 4, !tbaa !1
  %sub = sub nsw i32 1, %0
  %xor = xor i32 %sub, %0
  %tobool = icmp ne i32 %sub, %0
  %xor. = select i1 %tobool, i32 %xor, i32 1
  ret i32 %xor.
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_341/foo.c -o reduce_341/foo.bc

C source code:

int a;
int fn1() { return 1 - a ^ a ?: 1; }

x86-64 from LLVM:

fn1:                                    # @fn1
	movl	a(%rip), %ecx
	movl	$1, %edx
	movl	$1, %esi
	subl	%ecx, %esi
	movl	%esi, %eax
	xorl	%ecx, %eax
	cmpl	%ecx, %esi
	cmovel	%edx, %eax
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_341/foo.c -S -o -

x86-64 from GCC:

fn1:
	movl	a(%rip), %edx
	movl	$1, %eax
	movl	%eax, %ecx
	subl	%edx, %ecx
	xorl	%ecx, %edx
	cmovne	%edx, %eax
	ret

COMMAND: gcc -w -O3 reduce_341/foo.c -S -o -