AMDGPU/SI: Fix s_waitcnt insertion for flat instructions

Summary:
This was broken in r260694 which swapped the address and data operands
for flat store instructions.  The code in SIInsertWaits assumes
that the data operand always comes before the address operand, so
we need to add a special case for flat.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D17366

llvm-svn: 261330