[AMDGPU] TableGen: change individual instruction flags to bit type from bits<1>

Summary: This is needed to be able to use this flags in InstrMappings.

Reviewers: tstellarAMD, vpykhtin

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye

Differential Revision: https://reviews.llvm.org/D26666

llvm-svn: 286960