AMDGPU: Make various NamedOperands upper case

Summary:
Avoid name clashes with the corresponding bit fields in the instruction
encoding.

Change-Id: Id1644e703e976e78f7af93788d9f44cb48c3251f

Reviewers: arsenm, rampitec, kzhuravl

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

Differential Revision: https://reviews.llvm.org/D47433

llvm-svn: 333905