[AArch64][GlobalISel] Fold in G_ANYEXT/G_ZEXT into TB(N)Z

This is similar to the code in getTestBitOperand in AArch64ISelLowering. Instead
of implementing all of the TB(N)Z optimizations at once, this patch implements
the simplest case first. The way that this is set up should make it fairly easy
to add the rest as we go along.

The idea here is that after determining that we can use a TB(N)Z, we can
continue looking through instructions and perform further folding.

In this case, when we have a G_ZEXT or G_ANYEXT where the extended bits are not
used, we can fold it into the TB(N)Z.

Differential Revision: https://reviews.llvm.org/D73673