ISLE: Fix a bug with extractor ordering (#4661)

https://github.com/bytecodealliance/wasmtime/pull/4661

Co-authored-by: Chris Fallin <chris@cfallin.org>

cranelift/isle/isle/isle_examples/run/extractor_ordering_bug.isle

@@ -0,0 +1,19 @@
+
+(type u32 (primitive u32))
+
+(decl identity (u32) u32)
+(extern extractor infallible identity identity)
+
+(decl is_zero (u32) u32)
+(extern extractor is_zero is_zero)
+
+(decl test (u32) u32)
+
+;; This exposes a bug where infallible extractors were running before fallible
+;; ones, as the derived ordering for the `Extract` type was ordering them ahead
+;; of the fallible ones. The result is that the fallible `is_zero` extractor
+;; never runs, as the `identity` extractor will always succeed before it's
+;; called.
+(rule (test (identity x)) x)
+
+(rule (test (is_zero x)) 2)

cranelift/isle/isle/isle_examples/run/extractor_ordering_bug_main.rs

@@ -0,0 +1,23 @@
+mod extractor_ordering_bug;
+
+struct Context;
+impl extractor_ordering_bug::Context for Context {
+    fn is_zero(&mut self, val: u32) -> Option<u32> {
+        if val == 0 {
+            Some(val)
+        } else {
+            None
+        }
+    }
+
+    fn identity(&mut self, val: u32) -> u32 {
+        val
+    }
+}
+
+fn main() {
+    let mut ctx = Context;
+
+    assert_eq!(extractor_ordering_bug::constructor_test(&mut ctx, 0), Some(2));
+    assert_eq!(extractor_ordering_bug::constructor_test(&mut ctx, 1), Some(1));
+}