ISLE: reject multi-term rules with explicit priorities (#5663)

In multi-terms, all matching rules fire. We treat the result as an
unordered set of values, so setting rule priorities is meaningless. We
want to prohibit relying on the rule match order in this case.

Also, codegen can produce invalid Rust if rules with different
priorities both match against a multi-term. We first documented this
symptom in #5647. As far as I can figure, prohibiting rule priorities
prevents all possible instances of that bug.

At some point in the future we might decide we want to carefully define
semantics for multi-term result ordering, at which point we can revisit
this.

cranelift/isle/isle/tests/run_tests.rs

@@ -15,8 +15,12 @@ pub fn run_pass(filename: &str) {
 }
 
 pub fn run_fail(filename: &str) {
-    if build(filename).is_ok() {
-        panic!("test {} passed unexpectedly", filename);
+    match build(filename) {
+        Ok(_) => panic!("test {} passed unexpectedly", filename),
+        Err(err) => {
+            // Log the actual errors for use with `cargo test -- --nocapture`
+            println!("failed, as expected:\n{:?}", err);
+        }
     }
 }