additional simplifications to avoid spurious branching on select result

[jberthold]

Companion PR to runtimeverification/wasm-semantics#655 .
A simplification is required to avoid a spurious branch in the call_add* proofs

[ehildenb]

@jberthold typically we would add claim ... which exercise the simplifactions, in order to test that the actual simplification we need is correctly being applied. This claim can be produced by extracitng out the original condition/term you wanted simplified as the LHS, and the RHS is what you wanted it simplified to. Taht way, we continue to test that we don't accidentally undo simplifications we care about into the future.

@virgil-serbanuta can let you know where to put such claims in this repo.

[jberthold]

In order to get an example term that we want simplified, I tested the change to wasm-semantics with mx-backend's current master. It turned out that some other changes merged in the meanwhile seem to have removed the need for these simplifications. Closing for now