Souper says:

; Function: fn1
%0:i8 = var 
%1:i32 = sext %0
%2:i32 = addnsw 4294967168:i32, %1
%3:i1 = ne 0:i32, %2
cand %3 1:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_377/foo.bc

LLVM says:

define i32 @fn1() #0 {
entry:
  %0 = load i8* @a, align 1, !tbaa !1
  %conv = sext i8 %0 to i32
  %sub = add nsw i32 %conv, -128
  %tobool = icmp ne i32 %sub, 0
  %sub. = select i1 %tobool, i32 %sub, i32 1
  ret i32 %sub.
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_377/foo.c -o reduce_377/foo.bc

C source code:

char a;
int fn1() { return a - 128 ?: 1; }

x86-64 from LLVM:

fn1:                                    # @fn1
	movsbl	a(%rip), %ecx
	addl	$-128, %ecx
	movl	$1, %eax
	cmovnel	%ecx, %eax
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_377/foo.c -S -o -

x86-64 from GCC:

fn1:
	movsbl	a(%rip), %eax
	addl	$-128, %eax
	ret

COMMAND: gcc -w -O3 reduce_377/foo.c -S -o -