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.
We implement this by first rounding to nearest integer using the current
rouding mode, then converting this value from floating point to an integral
value.
Merry