[GlobalISel] Port some basic undef combines from DAGCombiner.cpp

This ports some combines from DAGCombiner.cpp which perform some trivial
transformations on instructions with undef operands.

Not having these can make it extremely annoying to find out where we differ
from SelectionDAG by looking at existing lit tests. Without them, we tend to
produce pretty bad code generation when we run into instructions which use
undef operands.

Also remove the nonpow2_store_narrowing testcase from arm64-fallback.ll, since
we no longer fall back on the add.

Differential Revision: https://reviews.llvm.org/D76339