Fix the (zext (zextload)) case to trigger, similarly for sign extends.

Fix the (zext (zextload)) case to trigger, similarly for sign extends.
Allow (zext (truncate)) to apply after legalize if the target supports
AND (which all do).

This compiles
short %foo() {
        %tmp.0 = load ubyte* %X         ; <ubyte> [#uses=1]
        %tmp.3 = cast ubyte %tmp.0 to short             ; <short> [#uses=1]
        ret short %tmp.3
}

to:
_foo:
        movzbl _X, %eax
        ret

instead of:

_foo:
        movzbl _X, %eax
        movzbl %al, %eax
        ret

thanks to Evan for pointing this out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24709 91177308-0d34-0410-b5e6-96231b3b80d8