Souper says:

; Function: fn2
%0:i8 = var 
%1:i32 = sext %0
%2:i32 = addnsw 4294966725:i32, %1
%3:i1 = slt %2, 0:i32
cand %3 1:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_549/foo.bc

LLVM says:

define i32 @fn1(i32 %p1) #0 {
entry:
  %cmp = icmp slt i32 %p1, 0
  %p1.lobit = lshr i32 %p1, 31
  %conv. = select i1 %cmp, i32 %p1.lobit, i32 0
  ret i32 %conv.
}
define void @fn2() #1 {
entry:
  %0 = load i8* @a, align 1, !tbaa !1
  %conv = sext i8 %0 to i32
  %sub = add nsw i32 %conv, -571
  %cmp.i = icmp slt i32 %sub, 0
  %p1.lobit.i = lshr i32 %sub, 31
  %conv..i = select i1 %cmp.i, i32 %p1.lobit.i, i32 0
  store i32 %conv..i, i32* @b, align 4, !tbaa !4
  ret void
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_549/foo.c -o reduce_549/foo.bc

C source code:

char a;
int b;
int fn1(p1) { return p1 < 0 ?: 0; }

void fn2() { b = fn1(a - 571); }

x86-64 from LLVM:

fn1:                                    # @fn1
	andl	%edi, %edi
	shrl	$31, %edi
	movl	%edi, %eax
	retq
fn2:                                    # @fn2
	movsbl	a(%rip), %eax
	addl	$-571, %eax             # imm = 0xFFFFFFFFFFFFFDC5
	andl	%eax, %eax
	shrl	$31, %eax
	movl	%eax, b(%rip)
	retq
.Ltmp1:

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_549/foo.c -S -o -

x86-64 from GCC:

fn1:
	movl	%edi, %eax
	shrl	$31, %eax
	ret
fn2:
.LFB1:
	movl	$1, b(%rip)
	ret
.LFE1:

COMMAND: gcc -w -O3 reduce_549/foo.c -S -o -