[X86] Minor cleanups to addShuffleComments in X86MCInstPrinter.cpp. NFCI

-Replace some ifs that should be impossible with asserts.
-Use X86::AddrDisp and X86::AddrNumOperands to make code more readable
-Use X86II::isKMasked/isKMergeMasked to do some operand skipping to remove or simplify switches