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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208329 91177308-0d34-0410-b5e6-96231b3b80d8