Handle i1/i8/i16 constant integer arguments to calls by prepromoting them.

Before we would bail out on i1 arguments all together, now we just bail on
non-constant ones.  Also, we used to emit extraneous code.  e.g. test12 was:

	movb	$0, %al
	movzbl	%al, %edi
	callq	_test12

and test13 was:
	movb	$0, %al
	xorl	%edi, %edi
	movb	%al, 7(%rsp)
	callq	_test13f

Now we get:

	movl	$0, %edi
	callq	_test12
and:
	movl	$0, %edi
	callq	_test13f

llvm-svn: 129751