Souper says:

; Function: fn1
%0:i32 = var 
%1:i64 = sext %0
%2:i64 = subnsw 2036854775807:i64, %1
%3:i1 = slt %2, 0:i64
cand %3 0:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_493/foo.bc

LLVM says:

define i32 @fn1() #0 {
entry:
  %0 = load i32* @a, align 4, !tbaa !1
  %conv = sext i32 %0 to i64
  %sub = sub nsw i64 2036854775807, %conv
  %cmp = icmp slt i64 %sub, 0
  %sub.lobit = lshr i64 %sub, 63
  %1 = trunc i64 %sub.lobit to i32
  %conv1. = select i1 %cmp, i32 %1, i32 0
  ret i32 %conv1.
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_493/foo.c -o reduce_493/foo.bc

C source code:

int a;
int fn1() { return 2036854775807 - a < 0 ?: 0; }

x86-64 from LLVM:

fn1:                                    # @fn1
	movslq	a(%rip), %rcx
	movabsq	$2036854775807, %rax    # imm = 0x1DA3E015FFF
	subq	%rcx, %rax
	shrq	$63, %rax
	andl	%eax, %eax
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_493/foo.c -S -o -

x86-64 from GCC:

fn1:
	xorl	%eax, %eax
	ret

COMMAND: gcc -w -O3 reduce_493/foo.c -S -o -