Only check for errors in predicate when skipping impl assembly

Prior to PR rust-lang#91205, checking for errors in the overall obligation
would check checking the `ParamEnv`, due to an incorrect
`super_visit_with` impl. With this bug fixed, we will now
bail out of impl candidate assembly if the `ParamEnv` contains
any error types.

In practice, this appears to be overly conservative - when an error
occurs early in compilation, we end up giving up early for some
predicates that we could have successfully evaluated without overflow.
By only checking for errors in the predicate itself, we avoid causing
additional spurious 'type annotations needed' errors after a 'real'
error has already occurred.

With this PR, the diagnostic changes caused by PR rust-lang#91205 are reverted.

tests/ui/crashes/ice-6252.stderr

@@ -21,15 +21,16 @@ LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
    |          |
    |          help: you might be missing a type parameter: `, VAL`
 
-error[E0283]: type annotations needed
-  --> $DIR/ice-6252.rs:10:12
+error[E0046]: not all trait items implemented, missing: `VAL`
+  --> $DIR/ice-6252.rs:10:1
    |
+LL |     const VAL: T;
+   |     ------------- `VAL` from trait
+...
 LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
-   |            ^^^^^^^^^^^^^^ cannot infer type for struct `Multiply<N, M>`
-   |
-   = note: cannot satisfy `Multiply<N, M>: TypeVal<usize>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `VAL` in implementation
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0283, E0412.
-For more information about an error, try `rustc --explain E0283`.
+Some errors have detailed explanations: E0046, E0412.
+For more information about an error, try `rustc --explain E0046`.