Complete cut-to-width for modular arithmetic

Complete cut-to-width for modular arithmetic

 For each modular argument, go through the nodes that provide its
 value and try to narrow down their bitwidth. If we fail on any and
 the result might be too wide, splice in an explicit call to LOGAND
 or MASK-SIGNED-FIELD. Skip that last step if the value is an argument
 to an equivalent LOGAND or MASK-SIGNED-FIELD.

 Test case by Eric Marsden.