[AArch64][GlobalISel] Use ZExtValue for zext(xor) when invert tb(n)z

Currently, we use SExtValue to decide whether to invert tbz or tbnz.
However, for the case zext (xor x, c), we should use ZExt rather
than SExt otherwise we will generate totally opposite branches.

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D108755