[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 Remove out-of-date comments for shouldBeEmitted() (filter() was removed)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