[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365512
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