[BOLT][NFC] Use getShortOpcodeArith in X86MCPlusBuilder

Unify `llvm::X86::getRelaxedOpcodeArith` and `getShortArithOpcode` in
X86MCPlusBuilder.cpp.

Addresses https://lists.llvm.org/pipermail/llvm-dev/2022-January/154526.html

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D121404