ISLE: Resolve overlap in prelude.isle and x64/inst.isle (#4941)

Resolve overlap in the ISLE prelude and the x64 inst module by introducing new types that allow better sharing of extractor resuls, or falling back on priorities.

cranelift/isle/isle/src/error.rs

@@ -125,9 +125,9 @@ impl std::fmt::Display for Error {
 
             Error::Errors(_) => write!(
                 f,
-                "found {} errors:\n\n{}",
+                "{}found {} errors",
+                DisplayErrors(self.unwrap_errors()),
                 self.unwrap_errors().len(),
-                DisplayErrors(self.unwrap_errors())
             ),
         }
     }