[GVN][VNCoercion] Remove load widening leftovers (NFCI)

GVN load widening was disabled in D24096. This removes various
support code that is no longer relevant.

The way this works nowadays is that we return PartialAlias with
an offset from BasicAA and this gets passed on as a clobber by
MDA. However, PartialAlias will only be returned if the load is
properly nested inside the other load.

This just removes the bulk of the code, but some additional
cleanup can be done here now that we don't need to distinguish
between load and store cases.