AMDGPU: Reduce the duration of whole-quad-mode

Summary:
This contains two changes that reduce the time spent in WQM, with the
intention of reducing bandwidth required by VMEM loads:

1. Sampling instructions by themselves don't need to run in WQM, only their
   coordinate inputs need it (unless of course there is a dependent sampling
   instruction). The initial scanInstructions step is modified accordingly.

2. When switching back from WQM to Exact, switch back as soon as possible.
   This affects the logic in processBlock.

This should always be a win or at best neutral.

There are also some cleanups (e.g. remove unused ExecExports) and some new
debugging output.

Reviewers: arsenm, tstellarAMD, mareko

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D22092

llvm-svn: 280590