Souper says:

; Function: fn1
%0:i32 = var 
%1:i1 = slt 4294967295:i32, %0
pc %1 1:i1
%2:i32 = xor 4294967295:i32, %0
%3:i1 = slt 4294967295:i32, %2
cand %3 0:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_236/foo.bc

LLVM says:

define void @fn1() #0 {
entry:
  %.pr = load i32* @a, align 4, !tbaa !1
  %cmp5 = icmp sgt i32 %.pr, -1
  br i1 %cmp5, label %for.cond1thread-pre-split.lr.ph, label %for.end3

for.cond1thread-pre-split.lr.ph:                  ; preds = %entry
  %.pr4 = load i32* @b, align 4, !tbaa !1
  %tobool = icmp eq i32 %.pr4, 0
  br i1 %tobool, label %for.inc.split.us.preheader, label %for.cond1.preheader

for.cond1.preheader:                              ; preds = %for.cond1thread-pre-split.lr.ph
  br label %for.cond1

for.inc.split.us.preheader:                       ; preds = %for.cond1thread-pre-split.lr.ph
  %0 = xor i32 %.pr, -1
  %1 = icmp sgt i32 %0, -1
  %smax = select i1 %1, i32 %0, i32 -1
  %2 = add i32 %.pr, %smax
  %3 = add i32 %2, 2
  %end.idx = add i32 %2, 2
  %n.vec = and i32 %3, -8
  %cmp.zero = icmp eq i32 %n.vec, 0
  %rev.ind.end = sub i32 %.pr, %n.vec
  br i1 %cmp.zero, label %middle.block, label %vector.body.preheader

vector.body.preheader:                            ; preds = %for.inc.split.us.preheader
  br label %vector.body

vector.body:                                      ; preds = %vector.body.preheader, %vector.body
  %index = phi i32 [ %index.next, %vector.body ], [ 0, %vector.body.preheader ]
  %index.next = add i32 %index, 8
  %4 = icmp eq i32 %index.next, %n.vec
  br i1 %4, label %middle.block.loopexit, label %vector.body, !llvm.loop !5

middle.block.loopexit:                            ; preds = %vector.body
  br label %middle.block

middle.block:                                     ; preds = %middle.block.loopexit, %for.inc.split.us.preheader
  %resume.val = phi i32 [ %.pr, %for.inc.split.us.preheader ], [ %rev.ind.end, %middle.block.loopexit ]
  %new.indc.resume.val = phi i32 [ 0, %for.inc.split.us.preheader ], [ %n.vec, %middle.block.loopexit ]
  %cmp.n = icmp eq i32 %end.idx, %new.indc.resume.val
  br i1 %cmp.n, label %for.cond.for.end3_crit_edge, label %for.inc.split.us.preheader8

for.inc.split.us.preheader8:                      ; preds = %middle.block
  br label %for.inc.split.us

for.inc.split.us:                                 ; preds = %for.inc.split.us.preheader8, %for.inc.split.us
  %dec6.us = phi i32 [ %dec.us, %for.inc.split.us ], [ %resume.val, %for.inc.split.us.preheader8 ]
  %dec.us = add nsw i32 %dec6.us, -1
  %cmp.us = icmp sgt i32 %dec6.us, 0
  br i1 %cmp.us, label %for.inc.split.us, label %for.cond.for.end3_crit_edge.loopexit, !llvm.loop !8

for.cond1:                                        ; preds = %for.cond1.preheader, %for.cond1
  br label %for.cond1

for.cond.for.end3_crit_edge.loopexit:             ; preds = %for.inc.split.us
  br label %for.cond.for.end3_crit_edge

for.cond.for.end3_crit_edge:                      ; preds = %for.cond.for.end3_crit_edge.loopexit, %middle.block
  store i32 -1, i32* @a, align 4, !tbaa !1
  br label %for.end3

for.end3:                                         ; preds = %for.cond.for.end3_crit_edge, %entry
  ret void
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_236/foo.c -o reduce_236/foo.bc

C source code:

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

x86-64 from LLVM:

fn1:                                    # @fn1
	movl	a(%rip), %eax
	testl	%eax, %eax
	js	.LBB0_10
	cmpl	$0, b(%rip)
	je	.LBB0_2
.LBB0_11:                               # %for.cond1
	jmp	.LBB0_11
.LBB0_2:                                # %for.inc.split.us.preheader
	movl	%eax, %esi
	notl	%esi
	cmpl	$-2, %esi
	movl	$-1, %r9d
	movl	$-1, %ecx
	cmovgl	%esi, %ecx
	leal	2(%rax,%rcx), %r10d
	movl	%r10d, %ecx
	andl	$-8, %ecx
	xorl	%edi, %edi
	movl	%r10d, %edx
	andl	$-8, %edx
	je	.LBB0_6
	movl	%eax, %r8d
	subl	%ecx, %r8d
	cmpl	$-2, %esi
	cmovgl	%esi, %r9d
	leal	2(%rax,%r9), %eax
	andl	$-8, %eax
.LBB0_4:                                # %vector.body
	addl	$-8, %eax
	jne	.LBB0_4
	movl	%r8d, %eax
	movl	%edx, %edi
.LBB0_6:                                # %middle.block
	cmpl	%edi, %r10d
	je	.LBB0_9
	incl	%eax
.LBB0_8:                                # %for.inc.split.us
	decl	%eax
	testl	%eax, %eax
	jg	.LBB0_8
.LBB0_9:                                # %for.cond.for.end3_crit_edge
	movl	$-1, a(%rip)
.LBB0_10:                               # %for.end3
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_236/foo.c -S -o -

x86-64 from GCC:

fn1:
	movl	a(%rip), %edx
	testl	%edx, %edx
	js	.L1
	movl	b(%rip), %eax
	testl	%eax, %eax
	je	.L3
.L4:
	jmp	.L4
.L3:
	movl	$-1, a(%rip)
.L1:
	rep ret

COMMAND: gcc -w -O3 reduce_236/foo.c -S -o -