[DAG] mergeTruncStores - early out if we collect more than the maximum number of stores

If we have an excessive number of stores in a single chain then the candidate WideVT may exceed the maximum width of an EVT integer type (and will assert) - but since mergeTruncStores doesn't support anything wider than a i64 store we should just early-out if we've collected more than stores than that.

Fixes #63306