AMDGPU: Stop using .NAME in .td files

Summary:
.NAME is a bit of an odd duck, in that we should really treat it like
a template argument, but we currently don't, and so when and where
NAME is initialized and how is pretty inconsistent. Best to just avoid
using it as a field of already instantiated records, and use cast to
string instead.

Change-Id: I5a0c202401cede3d5c3827ab9c7858ea48b29108

Reviewers: arsenm, rampitec

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

Differential Revision: https://reviews.llvm.org/D43551

llvm-svn: 325794