Fix LocalCSE's usable local selection (#2638)

Now that we have subtypes, we cannot reuse any local that contains the
same expression, because that local's type can be a supertype. For
example:
```
(local $0 anyref)
(local $1 nullref)
...
(local.set $0 (ref.null))
(local.set $1 (ref.null)) ;; cannot be replaced with (local.get $0)
```

This extends `usables` map's key to contain both `HashedExpression` and
the local's type, so we can get the right usable local in presence of
subtypes.

2 files changed, 38 insertions, 3 deletions

diff --git a/test/passes/flatten_local-cse_all-features.txt b/test/passes/flatten_local-cse_all-features.txt
index 0697e27a5..fc2dff3aa 100644
--- a/test/passes/flatten_local-cse_all-features.txt
+++ b/test/passes/flatten_local-cse_all-features.txt
@@ -762,6 +762,7 @@
  )
 )
 (module
+ (type $none_=>_none (func))
  (type $none_=>_funcref (func (result funcref)))
  (func $subtype-test (; 0 ;) (result funcref)
   (local $0 nullref)
@@ -789,4 +790,23 @@
    (local.get $2)
   )
  )
+ (func $test (; 1 ;)
+  (local $0 anyref)
+  (local $1 nullref)
+  (local $2 nullref)
+  (block $label$1
+   (local.set $0
+    (ref.null)
+   )
+   (local.set $1
+    (ref.null)
+   )
+  )
+  (local.set $2
+   (local.get $1)
+  )
+  (drop
+   (local.get $1)
+  )
+ )
 )
diff --git a/test/passes/flatten_local-cse_all-features.wast b/test/passes/flatten_local-cse_all-features.wast
index 6ca154278..112dfe647 100644
--- a/test/passes/flatten_local-cse_all-features.wast
+++ b/test/passes/flatten_local-cse_all-features.wast
@@ -288,14 +288,29 @@
  )
 )
 
-;; After --flatten, there will be a series of chain copies between multiple
-;; locals, but some of the locals will be nullref type and others funcref type.
-;; We cannot make locals of different types a common subexpression.
 (module
+ ;; After --flatten, there will be a series of chain copies between multiple
+ ;; locals, but some of the locals will be nullref type and others funcref type.
+ ;; We cannot make locals of different types a common subexpression.
  (func $subtype-test (result funcref)
   (nop)
   (loop $label$1 (result nullref)
    (ref.null)
   )
  )
+
+ (func $test
+  (local $0 anyref)
+  (drop
+   (block $label$1 (result nullref)
+    (local.set $0
+     (ref.null)
+    )
+    ;; After --flatten, this will be assigned to a local of nullref type. After
+    ;; --local-cse, even if we set (ref.null) to local $0 above, this should not
+    ;; be replaced with $0, because it is of type anyref.
+    (ref.null)
+   )
+  )
+ )
 )