It seems our `compile` fuzz target for ISLE has not been regularly
tested, as it was never updated for the `isle` -> `cranelift_isle` crate
renaming. This PR fixes it to compile again.
This also includes a simple fix in the typechecking: when verifying that
a term decl is valid, we might insert a term ID into the name->ID map
before fully checking that all of the types exist, and then skipping
(for error recovery purposes) the actual push onto the term-signature
vector if one of the types does have an error. This phantom TID can
later cause a panic. The fix is to avoid adding to the map until we have
fully verified the term decl.
56236fb58e