diffCallSites always built an AssumptionContext, so call sites made
optimistic equivalence assumptions even when the caller disabled them.
This made matchForBlockDiff over-match, and the re-check in unify() then
hit the "structural differences second time around?" assertion.
Thread the caller's AssumptionContext into diffCallSites so call sites
honor the no-assumptions request like every other instruction kind.
Fixes#184133