[AMDGPU] Created a sub-register class for the return address operand in the return instruction.

Function return instruction lowering, currently uses the fixed register pair s[30:31] for holding
the return address. It can be any SGPR pair other than the CSRs. Created an SGPR pair sub-register class
exclusive of the CSRs, and used this regclass while lowering the return instruction.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D63924

llvm-svn: 365512