generalize the previous code to use the full generality of LEA

for i32/i64 expressions (we could also do i16 on cpus where
i16 lea is fast, but I didn't add this).  On the example, we now
generate:

_test:
	movl	4(%esp), %eax
	cmpl	$42, (%eax)
	setl	%al
	movzbl	%al, %eax
	leal	4(%eax,%eax,8), %eax
	ret

instead of:

_test:
	movl	4(%esp), %eax
	cmpl	$41, (%eax)
	movl	$4, %ecx
	movl	$13, %eax
	cmovg	%ecx, %eax
	ret

llvm-svn: 66869