[X86][NFC] Move table from getRelaxedOpcodeArith into its own class
Move out the table and prepare the code to reuse it for the reverse mapping.
Follows the example of memory folding/unfolding tables in X86InstrFoldTables.cpp
Preparation step to unify `llvm::X86::getRelaxedOpcodeArith` and
`getShortArithOpcode` in BOLT X86MCPlusBuilder.cpp.
Addresses https://lists.llvm.org/pipermail/llvm-dev/2022-January/154526.html
Reviewed By: skan, MaskRay
Differential Revision: https://reviews.llvm.org/D121402