[X86][NFC] Refactor X86InstrArithmetic.td by class

1. Extract the common code of some instructions into a class to reduce duplication
2. Refine some comments to the make the description of the class clearer

By this way, the records defined here will be consistent and easier to
maintain, I think.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D144154