Add TODO

cranelift/isle/TODO

@@ -0,0 +1,7 @@
+- Convert series of if-lets from MatchVariants into a match { ... }.
+  - Should be pretty simple; just need to recognize adjacent edges in
+    priority-order that are all MatchVariants on the same input.
+
+- Document semantics carefully, especially wrt extractors.
+
+- Verify that priorities work as expected.