R600/SI: Legalize CopyToReg during instruction selection

The instruction emitter will crash if it encounters a CopyToReg
node with a non-register operand like FrameIndex.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219428 91177308-0d34-0410-b5e6-96231b3b80d8