[GlobalISel] Remove an erroneous oneuse check in the G_ADD reassociation combine.

This check was unnecessary/incorrect, it was already being done by the target
hook default implementation, and the one in the matcher was checking for a
completely different thing. This change:
 1) Removes the check and updates affected tests which now do some more reassociations.
 2) Modifies the AMDGPU hooks which were stubbed with "return true" to also do the oneuse
    check. Not sure why I didn't do this the first time.