AMDGPU/SI: Don't use non-0 waitcnt values when waiting on Flat instructions

Summary:
Flat instruction can return out of order, so we need always need to wait
for all the outstanding flat operations.

Reviewers: tony-tye, arsenm

Subscribers: kzhuravl, wdng, nhaehnle, llvm-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D25998

llvm-svn: 285479