[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