[AMDGPU] Prevent sequences of non-instructions disrupting GCNHazardRecognizer wait state counting

Summary:
This fixes a bug where a large number of implicit def instructions can fill the GCNHazardRecognizer lookahead buffer causing required NOPs to not be inserted.

Reviewers: nhaehnle, arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D51726

Change-Id: Ie75338f94de704ee5816b05afd0c922c6748a95b
llvm-svn: 341798