AMDGPU/SI: Use a better method for determining the largest pressure sets

Summary:
There are a few different sgpr pressure sets, but we only care about
the one which covers all of the sgprs.  We were using hard-coded
register pressure set names to determine the reg set id for the
biggest sgpr set.  However, we were using the wrong name, and this
method is pretty fragile, since the reg pressure set names may
change.

The new method just looks for the pressure set that contains the most
reg units and sets that set as our SGPR pressure set.  We've also
adopted the same technique for determining our VGPR pressure set.

Reviewers: arsenm

Subscribers: MatzeB, arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23687

llvm-svn: 279867