aarch64: move bogus assertion

aarch64: move bogus assertion

Asserting "idx" to be non-negative when subsequent code handles this
case is bogus. In fact the assertion triggers e.g. when mistakenly
using the arm32 comment character @ following an instruction.

While doing this I also noticed that despite there being local
variables "detail" and "idx", not all places where they could be used
did actually make use of them, so this is being adjusted at once.

Finally, for the code to be slightly more robust, also change
comparisons against -1 to such checking for a (non-)negative value.