Heavily simplify logical immediate encoding.
This is based on the observation that if a valid repeating element
exists, it repeats through `value`. Thus it does not matter which
one you analyse. Thus we skip over the least significent element
if LSB = 1 by masking it out with `inverse_mask_from_trailing_ones`,
to avoid the degenerate case of a stretch of 1 bits going 'round
the end' of the word.