[RS4GC] Handle special cases in unreachable code for memcpy/memmov

The existing code doesn't expect dummy values (undef, poison, null-derived
constants etc) as arguments of these intrinsics. However, they can be there
in unreached code. Currently we fail trying to find base for them.

Handle these cases separately. Return null as base for them to be consistent
with the handling in the main algorithm in findBaseDefiningValue.

Differential Revision: https://reviews.llvm.org/D129561
Reviewed By: apilipenko