[VNCoercion] Disallow coercion between different ni addrspaces

I'm not sure if it would be legal by the IR reference to introduce
an addrspacecast here, since the IR reference is a bit vague on
the exact semantics, but at least for our usage of it (and I
suspect for many other's usage) it is not. For us, addrspacecasts
between non-integral address spaces carry frontend information that the
optimizer cannot deduce afterwards in a generic way (though we
have frontend specific passes in our pipline that do propagate
these). In any case, I'm sure nobody is using it this way at
the moment, since it would have introduced inttoptrs, which
are definitely illegal.

Fixes PR38375

Co-authored-by: Keno Fischer <keno@alumni.harvard.edu>

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D50010