[X86][tablgen] Refine the class RecognizableInstr. NFCI

1. Add comments to explain why we set `isAsmParserOnly` for XACQUIRE and XRELEASE
2. Check `X86Inst` in the constructor of `RecognizableInstrBase` so that
   we can avoid the case where one of it's field is not initialized but
   accessed by user. (e.g. in X86EVEX2VEXTablesEmitter.cpp)
3. Move `Rec` from `RecognizableInstrBase` to `RecognizableInstr` to reduce
   size of `RecognizableInstrBase`
4. Remove out-of-date comments for shouldBeEmitted() (filter() was removed)
5. Add a basic field `IsAsmParserOnly` and remove the field
   `ShouldBeEmitted` b/c we can deduce it w/ little overhead