[gvn] Handle a corner case w/vectors of non-integral pointers

If we try to coerce a vector of non-integral pointers to a narrower type (either narrower vector or single pointer), we use inttoptr and violate the semantics of non-integral pointers.  In theory, we can handle many of these cases, we just need to use a different code idiom to convert without going through inttoptr and back.

This shows up as wrong code bugs, and in some cases, crashes due to failed asserts.  Modeled after a change which has lived downstream for a couple years, though completely rewritten to be more idiomatic.