[GlobalISel] Only merge memory ops for mayLoad or mayStore instrs.

Summary:
We only need to merge memory operands for instructions that access
 memory. This slightly reduces the number of actions executed. 

Reviewers: MatzeB, rovka, dsanders

Reviewed By: dsanders

Subscribers: aemerson, igorb, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D36151

llvm-svn: 309944