[ARM64] Re-work parsing of ADD/SUB shifted immediate operands

The parsing of ADD/SUB shifted immediates needs to be done explicitly so
that better diagnostics can be emitted, as a side effect this also
removes some of the hacks in the current method of handling this operand
type.

Additionally remove manual CMP aliasing to ADD/SUB and use InstAlias
instead.

llvm-svn: 208329