ARM64: merge "extend" and "shift" addressing-mode enums.

In terms of assembly, these have too much overlap to be neatly modelled as
disjoint classes: in many cases "lsl" is an acceptable alternative to either
"uxtw" or "uxtx".
ARM64: remove dead validation code from the AsmParser.

If this code triggers, any immediate has already been validated so it can't
possibly trigger a diagnostic.