buffer-deallocation: consider aliases introduced by arith.select.

Currently, buffer deallocation considers arith.select to be
non-aliasing, which results in deallocs being inserted incorrectly. Since
arith.select doesn't implement any useful interfaces, this change just handles
it explicitly. Eventually this should probably be fixed properly, if this pass
is going to be used long term.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D132460