[X86][NFC] Fix a bug after D150436 and update some comments

D150436 changed the order for INC*r, but unintentionally remove 'isConvertibleToThreeAddress = 1' for INC*r_alt.
This patch also update wrong comment X86InstrArithmetic.td, and move class ITy to InstrInfo.td for it's a general class.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D151527