Always simplify call demands
Call demands satisfy the semantic equality
CU(...CU(U)) === U. In contrast to product demands (where we lose boxity information, #18907), we don't lose any information by doing this simplification and we gain less clutter: If we manage to reduce a demand to
topDmd), we won't display it in dumps and also have marginally smaller interface files.
So, do it!
There should be plenty of regression tests affected by this, so I don't think we need a new one for this.