[PowerPC] Handle constant zero bits in BitPermutationSelector

We currently crash when analyzing an AssertZExt node that has some bits that are
constant zeros (i.e. as a result of an and with a constant).

This issue was reported in https://bugs.llvm.org/show_bug.cgi?id=41088 and this
patch fixes that.

Differential revision: https://reviews.llvm.org/D72038