[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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234766
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