[PowerPC] [NFC] Create a helper function to copy register to particular...

[PowerPC] [NFC] Create a helper function to copy register to particular register class at PPCFastISel

Make copy register code as common function as following.

unsigned copyRegToRegClass(const TargetRegisterClass *ToRC, unsigned SrcReg, unsigned Flag = 0, unsigned SubReg = 0);

Differential Revision: https://reviews.llvm.org/D57368

llvm-svn: 352596