Insert explicit cut to width when needed

Insert explicit cut to width when needed

 When modular arithmetic operations are replaced with specialised
 modular variants, the result's bitwidth is determined by the variant,
 and might be wider than expected. If necessary, insert an explicit
 cut to the exact bitwidth before returning a value in a non-modular
 context.

 Spotted by pfdietz's random tester.

 Fixes lp#1199428.