Souper says:

; Function: fn1
%0:i32 = var 
%1:i1 = ne 0:i32, %0
%2:i32 = var 
%3:i1 = slt 4294967295:i32, %2
%4:i1 = and %1, %3
pc %4 1:i1
%5:i1 = slt %2, 0:i32
cand %5 0:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_701/foo.bc

LLVM says:

define void @fn1() #0 {
entry:
  %0 = load i32* @b, align 4, !tbaa !1
  %tobool = icmp ne i32 %0, 0
  %1 = load i32* @a, align 4
  %cmp = icmp sgt i32 %1, -1
  %or.cond = and i1 %tobool, %cmp
  br i1 %or.cond, label %for.inc.preheader, label %if.end

for.inc.preheader:                                ; preds = %entry
  %2 = icmp slt i32 %1, 0
  %.op = add i32 %1, -1
  %3 = select i1 %2, i32 %.op, i32 -1
  store i32 %3, i32* @a, align 4, !tbaa !1
  br label %if.end

if.end:                                           ; preds = %for.inc.preheader, %entry
  ret void
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_701/foo.c -o reduce_701/foo.bc

C source code:

int a, b;
void fn1() {
  if (b)
    for (; a >= 0; a = a - 1)
      ;
}

x86-64 from LLVM:

fn1:                                    # @fn1
	cmpl	$0, b(%rip)
	je	.LBB0_3
	movl	a(%rip), %eax
	testl	%eax, %eax
	js	.LBB0_3
	leal	-1(%rax), %ecx
	testl	%eax, %eax
	movl	$-1, %eax
	cmovsl	%ecx, %eax
	movl	%eax, a(%rip)
.LBB0_3:                                # %if.end
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_701/foo.c -S -o -

x86-64 from GCC:

fn1:
	movl	b(%rip), %edx
	testl	%edx, %edx
	je	.L1
	movl	a(%rip), %eax
	testl	%eax, %eax
	js	.L1
	movl	$-1, a(%rip)
.L1:
	rep ret

COMMAND: gcc -w -O3 reduce_701/foo.c -S -o -