Get ADCE to handle OpCopyMemory

ADCE does not treat OpCopyMemory as an instruction that references
memory.  Because of that stores are removed that should not be.

This change teaches ADCE that OpCopyMemory and OpCopyMemorySize both
loads from and stores to memory.  This will keep other stores live when
needed, and will allows ADCE to remove OpCopyMemory instructions as
well.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1556.