Implement rdar://7415680 - Twine integer support lacks greatness

Microoptimize Twine's with unsigned and int to not pin their value to
the stack.  This saves stack space in common cases and allows mem2reg
in the caller.  A simple example is:

void foo(const Twine &);
void bar(int x) {
  foo("xyz: " + Twine(x));
}

Before:

__Z3bari:
	subq	$40, %rsp
	movl	%edi, 36(%rsp)
	leaq	L_.str3(%rip), %rax
	leaq	36(%rsp), %rcx
	leaq	8(%rsp), %rdi
	movq	%rax, 8(%rsp)
	movq	%rcx, 16(%rsp)
	movb	$3, 24(%rsp)
	movb	$7, 25(%rsp)
	callq	__Z3fooRKN4llvm5TwineE
	addq	$40, %rsp
	ret

After:

__Z3bari:
	subq	$24, %rsp
	leaq	L_.str3(%rip), %rax
	movq	%rax, (%rsp)
	movslq	%edi, %rax
	movq	%rax, 8(%rsp)
	movb	$3, 16(%rsp)
	movb	$7, 17(%rsp)
	leaq	(%rsp), %rdi
	callq	__Z3fooRKN4llvm5TwineE
	addq	$24, %rsp
	ret

It saves 16 bytes of stack and one instruction in this case.

llvm-svn: 103107