AMDGPU/GlobalISel: Implement select() for G_FCONSTANT

Summary: Also clean up G_CONSTANT selection.

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D46170

llvm-svn: 332379