Fix non-determinism in Reassociate caused by address coincidences

[lizhengxing]

This PR pulls the upstream change, Fix non-determinism in Reassociate caused by address coincidences (llvm/llvm-project@ef8761f), into DXC.

Here's the summary of the change:

Between building the pair map and querying it there are a few places that erase and create Values. It's rare but the address of these newly created Values is occasionally the same as a
just-erased Value that we already have in the pair map. These coincidences should be accounted for to avoid non-determinism.

Thanks to Roman Tereshin for the test case.

This is part 6 (the last part) of the fix for #6659.