ISLE: Always default the priority to 0 (#4983)

* Always default priorities to 0 in the ISLE IR
* Fix a hidden overlap in the x64 backend

cranelift/isle/isle/src/sema.rs

@@ -431,8 +431,8 @@ pub struct Rule {
     /// The right-hand side expression that this rule evaluates upon successful
     /// match.
     pub rhs: Expr,
-    /// The priority of this rule, if any.
-    pub prio: Option<i64>,
+    /// The priority of this rule, defaulted to 0 if it was missing in the source.
+    pub prio: i64,
     /// The source position where this rule is defined.
     pub pos: Pos,
 }
@@ -1394,7 +1394,7 @@ impl TermEnv {
                         lhs,
                         iflets,
                         rhs,
-                        prio: rule.prio,
+                        prio: rule.prio.unwrap_or(0),
                         pos,
                     });
                 }

cranelift/isle/isle/src/trie.rs

@@ -337,7 +337,7 @@ impl<'a> TermFunctionsBuilder<'a> {
     fn build(&mut self) {
         for rule in 0..self.termenv.rules.len() {
             let rule = RuleId(rule);
-            let prio = self.termenv.rules[rule.index()].prio.unwrap_or(0);
+            let prio = self.termenv.rules[rule.index()].prio;
 
             let (pattern, expr) = lower_rule(self.typeenv, self.termenv, rule);
             let root_term = self.termenv.rules[rule.index()].lhs.root_term().unwrap();