aarch64: Tweak priorities of parsing-related errors

aarch64: Tweak priorities of parsing-related errors

There are three main kinds of error reported during parsing,
in increasing order of priority:

- AARCH64_OPDE_RECOVERABLE (register seen instead of immediate)
- AARCH64_OPDE_SYNTAX_ERROR
- AARCH64_OPDE_FATAL_SYNTAX_ERROR

This priority makes sense when comparing errors reported against the
same operand.  But if we get to operand 3 (say) and see a register
instead of an immediate, that's likely to be a better match than
something that fails with a syntax error at operand 1.

The idea of this patch is to prioritise parsing-related errors
based on operand index first, then by error code.  Post-parsing
errors still win over parsing errors, and their relative priorities
don't change.