[IR] Fix mayReadFromMemory() for writeonly calls

Current implementation of Instruction::mayReadFromMemory()
returns !doesNotAccessMemory() which is !ReadNone. This
does not take into account that the writeonly attribute
also indicates that the call does not read from memory.

The patch changes the predicate to !doesNotReadMemory()
that reflects the intended behavior.

Differential Revision: https://reviews.llvm.org/D69086

llvm-svn: 375389