R600/SI: Set hasSideEffects = 0 on load and store instructions.

Assuming unmodeled side effects interferes with some scheduling
opportunities.

Don't put it in the base class of DS instructions since there
are a few weird effecting, non load/store instructions there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222285 91177308-0d34-0410-b5e6-96231b3b80d8