[X86] Simplify the code for EFLAGS copy lowering, NFCI

1. MF.begin() == MF.end() -> MF.empty()
2. Set FlagsKilled by API modifiesRegister
3. Utilize APIs in X86GenMnemonicTables.inc to check arithmetic op
4. Merge duplicated code for rewrite*
5. Clang format

This is to address review comments in #91849