Two changes:

  * In promote32, if we can just promote a constant value, do so instead of
    promoting a constant dynamically.
  * In visitReturn inst, actually USE the promote32 argument that takes a
    Value*

The end result of this is that we now generate this:

test:
        mov %EAX, 0
        ret

instead of...

test:
        mov %AX, 0
        movzx %EAX, %AX
        ret

for:

ushort %test() {
        ret ushort 0
}

llvm-svn: 12679