[BasicAA] Compute minimal access extents lazily (#226931)
`aliasCheck` computes the minimal extent of an access before calling
`isObjectSmallerThan`, even though the latter immediately returns false
when the other pointer is not an identified object.
Move the extent computation behind the existing `isIdentifiedObject`
check. This preserves the result: whenever the extent can affect the
answer, the same value is computed and used as before; otherwise only
unused work is skipped.
