[AArch64][GlobalISel] Prefer mov for s32->s64 G_ZEXT

We can use an ORRWrs (mov) + SUBREG_TO_REG rather than a UBFX for G_ZEXT on
s32->s64.

This closer matches what SDAG does, and is likely more power efficient etc.

(Also fixed up arm64-rev.ll which had a fallback check line which was entirely
useless.)

Simple example: https://godbolt.org/z/h1jKKdx5c

Differential Revision: https://reviews.llvm.org/D102656