[RewriteStatepointsForGC] Strengthen assertions around liveness

This is related to the issues addressed in 234651.  These assertions check the properties ensured by that change at the place of use.  Note that a similiar property is checked in checkBasicSSA, but without the reachability constraint.  Technically, the liveness would be correct to include unreachable values, but this would be problematic for actual relocation.

llvm-svn: 234766