AMDGPU/GlobalISel: Mark 1-bit integer constants as legal

Summary:
These are mostly legal, but will probably need special lowering for some
cases.

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D33791

llvm-svn: 304628