[patch] Fix pr23046

[Richard Kenner]

     You'd V_C_E to a type with the same precision but with bounds not
     narrowed beyond what the bits can hold.  If that's the base type,
     fine, but something else if not.

Suppose the base type is signed SImode with precision of 32.  The subtype has
a range of 2-7, so is QImode, unsigned, and precision of 3.

So you are suggesting making a new type with what?  Unsigned QImode,
precision 8 and range 0-255?

How local would that type be?  Do we make one of such for each subtype
(meaning we'd likely have to find a place to stick it) or do we make a new
such type every time we need to do the operation?