[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309944 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed