[AMDGPU] SDWA: remove support for VOP2 instructions that have only 64-bit encoding

Summary:
Despite that this instructions are listed in VOP2, they are treated as VOP3 in specs. They should not support SDWA.
There are no real instructions for them, but there are pseudo instructions.

Reviewers: arsenm, vpykhtin, cfang

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye

Differential Revision: https://reviews.llvm.org/D34403

llvm-svn: 305999