These properties aren't additive. They are closer to ReadOnly and
WriteOnly. The default is ReadWrite. ReadMem cancels the write property and
WriteMem cancels the read property. Combining them leaves neither.
This patch checks that when we process WriteMem, the Mod flag is
still set. And for ReadMem we check that the Ref flag set still set.
I've updated 2 target intrinsics that were combining these properties.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D93571
