aarch64: Use constant fields in simple_index operands

aarch64: Use constant fields in simple_index operands

Update aarch64_{ins|ext}_simple_index to use constant fields, and swap
the order of the index and regno fields, so that the regno occupies the
last five bits.  (Splitting/combining a variable length value and a
fixed length value is easiest if the fixed length value occupies the
least significant bits.)