[AArch64][GlobalISel] Avoid creating redundant ubfx when selecting G_ZEXT

When selecting 32 b -> 64 b G_ZEXTs, we don't have to always emit the extend.

If the instruction feeding into the G_ZEXT implicitly zero extends the high
half of the register, we can just emit a SUBREG_TO_REG instead.

Differential Revision: https://reviews.llvm.org/D81897