[RFA:] Fix lsb? bug with insn fields beyond base insn size.

[Doug Evans]

Hans-Peter Nilsson writes:
 > The highest bit-number minus the lowest bit-number is the
 > number of bits less one.

Assuming one defines the field to be [low,high].
Another legitimate definition, within a localized context such
as intermediates used to produce some final result, is [low,high).
I'm sure there are other contexts where such a definition is legitimate.

 > If the field starts at bit 2 and ends
 > at bit 2 it's one bit long.

Such a field can also be written, setting aside bit numbering issues, [2,3).