AMDGPU/SI: Teach SIInstrInfo::FoldImmediate() to fold immediates into copies

Summary:
I put this code here, because I want to re-use it in a few other places.
This supersedes some of the immediate folding code we have in SIFoldOperands.
I think the peephole optimizers is probably a better place for folding
immediates into copies, since it does some register coalescing in the same time.

This will also make it easier to transition SIFoldOperands into a smarter pass,
where it looks at all uses of instruction at once to determine the optimal way to
fold operands.  Right now, the pass just considers one operand at a time.

Reviewers: arsenm

Subscribers: wdng, nhaehnle, arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23402

llvm-svn: 280744