[AArch64][GlobalISel] Walk through G_TRUNC in getTestBitReg

When you encounter a G_TRUNC, you are moving from a larger type to a smaller
type.

Asking for the i-th bit on a larger value is the same as asking for the i-th
bit on a smaller value.

So, we should always be able to walk through G_TRUNC when computing the bit
for a TB(N)Z.

Differential Revision: https://reviews.llvm.org/D73748