fold comparisons of gep'd alloca points with null to false,

implementing PR12013.  We now compile the testcase to:

__Z4testv:                              ## @_Z4testv
## BB#0:                                ## %_ZN4llvm15SmallVectorImplIiE9push_backERKi.exit
	pushq	%rbx
	subq	$64, %rsp
	leaq	32(%rsp), %rbx
	movq	%rbx, (%rsp)
	leaq	64(%rsp), %rax
	movq	%rax, 16(%rsp)
	movl	$1, 32(%rsp)
	leaq	36(%rsp), %rax
	movq	%rax, 8(%rsp)
	leaq	(%rsp), %rdi
	callq	__Z1gRN4llvm11SmallVectorIiLj8EEE
	movq	(%rsp), %rdi
	cmpq	%rbx, %rdi
	je	LBB0_2
## BB#1:
	callq	_free
LBB0_2:                                 ## %_ZN4llvm11SmallVectorIiLj8EED1Ev.exit
	addq	$64, %rsp
	popq	%rbx
	ret

instead of:

__Z4testv:                              ## @_Z4testv
## BB#0:
	pushq	%rbx
	subq	$64, %rsp
	xorl	%eax, %eax
	leaq	(%rsp), %rbx
	addq	$32, %rbx
	movq	%rbx, (%rsp)
	movq	%rbx, 8(%rsp)
	leaq	64(%rsp), %rcx
	movq	%rcx, 16(%rsp)
	je	LBB0_2
## BB#1:
	movl	$1, 32(%rsp)
	movq	%rbx, %rax
LBB0_2:                                 ## %_ZN4llvm15SmallVectorImplIiE9push_backERKi.exit
	addq	$4, %rax
	movq	%rax, 8(%rsp)
	leaq	(%rsp), %rdi
	callq	__Z1gRN4llvm11SmallVectorIiLj8EEE
	movq	(%rsp), %rdi
	cmpq	%rbx, %rdi
	je	LBB0_4
## BB#3:
	callq	_free
LBB0_4:                                 ## %_ZN4llvm11SmallVectorIiLj8EED1Ev.exit
	addq	$64, %rsp
	popq	%rbx
	ret

This doesn't shrink clang noticably though.

llvm-svn: 150944