AMDGPU/GlobalISel: Default to using TableGen'd instruction selector

Summary:
We can select all instructions that are marked as legal in a full piglit run,
so now is a good time to make the TableGen'd instruction selector default
for all opcodes.  This is NFC for a full piglit run, which is why there are
no tests.

Reviewers: arsenm, nhaehnle

Subscribers: kzhuravl, wdng, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D48198

llvm-svn: 335319