Defines enums for the LoongArch registers.
Adds the register class implementation for LoongArch.
Adds save and restore context functionality.
This only supports 64 bits integer and float-point register
implementation.
Fix https://github.com/llvm/llvm-project/issues/55398
Reviewed By: SixWeining
Differential Revision: https://reviews.llvm.org/D137010