GlobalISel/InstructionSelector: Implement GIR_CopyFConstantAsFPImm

Summary: AMDGPU will need this to enable the TableGen'd GlobalISel selector.

Reviewers: dsanders, aditya_nandakumar

Reviewed By: dsanders

Subscribers: rovka, kristof.beyls, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D45990

llvm-svn: 331579