InhabitedPredicate: avoid using a wildcard branch

This is error-prone. Explicitly write down which cases don't need
anything substituted. Turn the `OpaqueType` case, which currently
seems to be unreachable, into a `bug!`.

Author: meithecatte

compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs

@@ -265,7 +265,10 @@ impl<'tcx> InhabitedPredicate<'tcx> {
                 Some(InhabitedPredicate::True) => Some(InhabitedPredicate::True),
                 Some(a) => Some(a.or(tcx, b.instantiate_opt(tcx, args).unwrap_or(b))),
             },
-            _ => None,
+            Self::True | Self::False | Self::NotInModule(_) => None,
+            Self::OpaqueType(_) => {
+                bug!("unexpected OpaqueType in InhabitedPredicate");
+            }
         }
     }
 }