[MC] Store number of implicit operands in MCInstrDesc. NFC.

Combine the implicit uses and defs lists into a single list of uses
followed by defs. Instead of 0-terminating the list, store the number
of uses and defs. This avoids having to scan the whole list to find the
length and removes one pointer from MCInstrDesc (although it does not
get any smaller due to alignment issues).

Remove the old accessor methods getImplicitUses, getNumImplicitUses,
getImplicitDefs and getNumImplicitDefs as all clients are using the new
implicit_uses and implicit_defs.

Differential Revision: https://reviews.llvm.org/D142216