Souper says:

; Function: fn1
%0:i32 = var 
%1:i1 = eq 0:i32, %0
pc %1 1:i1
%2:i32 = addnsw 1:i32, %0
%3:i1 = slt %2, 6:i32
cand %3 1:i1

COMMAND: /home/regehr/souper/build/souper -stp-path=/usr/local/bin/stp reduce_75/foo.bc

LLVM says:

define void @fn1() #0 {
entry:
  store i32 0, i32* @a, align 4, !tbaa !1
  %b.promoted = load i32* @b, align 4, !tbaa !1
  br label %for.body

for.cond:                                         ; preds = %for.body
  store i32 %inc, i32* @a, align 4, !tbaa !1
  %cmp = icmp slt i32 %inc, 6
  br i1 %cmp, label %for.body, label %lbl_16

for.body:                                         ; preds = %entry, %for.cond
  %0 = phi i32 [ 0, %entry ], [ %inc, %for.cond ]
  %dec1 = phi i32 [ %b.promoted, %entry ], [ %dec, %for.cond ]
  %dec = add nsw i32 %dec1, -1
  %tobool = icmp eq i32 %0, 0
  %inc = add nsw i32 %0, 1
  br i1 %tobool, label %for.cond, label %lbl_16

lbl_16:                                           ; preds = %for.body, %for.cond
  %dec.lcssa = phi i32 [ %dec, %for.body ], [ %dec, %for.cond ]
  store i32 %dec.lcssa, i32* @b, align 4, !tbaa !1
  ret void
}

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -c -w -emit-llvm -O3 reduce_75/foo.c -o reduce_75/foo.bc

C source code:

int a, b;
void fn1() {
  a = 0;
  for (; a <= 5; a++) {
    --b;
    if (a)
      goto lbl_16;
  }
lbl_16:
  ;
}

x86-64 from LLVM:

fn1:                                    # @fn1
	movl	$0, a(%rip)
	movl	b(%rip), %edx
	decl	%edx
	xorl	%eax, %eax
.LBB0_2:                                # %for.body
	movl	%edx, %ecx
	testl	%eax, %eax
	jne	.LBB0_3
	incl	%eax
	movl	%eax, a(%rip)
	leal	-1(%rcx), %edx
	cmpl	$5, %eax
	jle	.LBB0_2
.LBB0_3:                                # %lbl_16
	movl	%ecx, b(%rip)
	retq

COMMAND: /home/regehr/souper/third_party/llvm/Debug/bin/clang -w -O3 reduce_75/foo.c -S -o -

x86-64 from GCC:

fn1:
	movl	b(%rip), %eax
	movl	$1, a(%rip)
	subl	$2, %eax
	movl	%eax, b(%rip)
	ret

COMMAND: gcc -w -O3 reduce_75/foo.c -S -o -