[AMDGPU] Add missing hazard for DPP-after-EXEC-write

Summary:
Following the docs, we need at least 5 wait states between an EXEC write
and an instruction that uses DPP.

Reviewers: tstellar, arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D34849

llvm-svn: 310013