[AArch64] Sink to umull if we know tops bits are zero.

This is an extension to the code for sinking splats to multiplies, where
if we can detect that the top bits are known-zero we can treat the
instruction like a zext. The existing code was also adjusted in the
process to make it more precise about only sinking if both operands are
zext or sext.

Differential Revision: https://reviews.llvm.org/D141275