[NVPTX] allow register copy between float and int

Summary:
Fixes PR24303. With Bruno's WIP (D11197) on PeepholeOptimizer, across-class
register copying (e.g. i32 to f32) becomes possible. Enhance
NVPTXInstrInfo::copyPhysReg to handle these cases.

Reviewers: jholewinski

Subscribers: eliben, jholewinski, llvm-commits, bruno

Differential Revision: http://reviews.llvm.org/D11622

llvm-svn: 243839