AMDGPU: Fix an interaction between WQM and polygon stippling

AMDGPU: Fix an interaction between WQM and polygon stippling

Summary:
This fixes a rare bug in polygon stippling with non-monolithic pixel shaders.

The underlying problem is as follows: the prolog part contains the polygon
stippling sequence, i.e. a kill. The main part then enables WQM based on the
_reduced_ exec mask, effectively undoing most of the polygon stippling.

Since we cannot know whether polygon stippling will be used, the main part
of a non-monolithic shader must always return to exact mode to fix this
problem.

Reviewers: arsenm, tstellarAMD, mareko

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23131

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280589 91177308-0d34-0410-b5e6-96231b3b80d8