AMDGPU/GlobalISel: Mark 32-bit float constants as legal

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D33212

llvm-svn: 304003