ISLE: fix compile fuzz target, and fix a simple error-reporting bug. (#3845)

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.

.github/workflows/main.yml

@@ -185,6 +185,8 @@ jobs:
     - run: cargo fetch
       working-directory: ./fuzz
     - run: cargo fuzz build --dev
+    # Check that the ISLE fuzz targets build too.
+    - run: cargo fuzz build --dev --fuzz-dir ./cranelift/isle/fuzz
 
   rebuild_isle:
     name: Rebuild ISLE