fix PR10605 / rdar://9930964 by adding a pretty scary missed check.

fix PR10605 / rdar://9930964 by adding a pretty scary missed check.
It's somewhat surprising anything works without this.  Before we would
compile the testcase into:

test:                                   # @test
	movl	$4, 8(%rdi)
	movl	8(%rdi), %eax
	orl	%esi, %eax
	cmpl	$32, %edx
	movl	%eax, -4(%rsp)          # 4-byte Spill
	je	.LBB0_2

now we produce:

test:                                   # @test
	movl	8(%rdi), %eax
	movl	$4, 8(%rdi)
	orl	%esi, %eax
	cmpl	$32, %edx
	movl	%eax, -4(%rsp)          # 4-byte Spill
	je	.LBB0_2

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137303 91177308-0d34-0410-b5e6-96231b3b80d8