Preserve exactness optimizing ref.cast + ref.as_non_null (#7605)

We would previously drop exactness when making the cast type
non-nullable.

Author: tlively

src/passes/OptimizeInstructions.cpp

@@ -2423,7 +2423,7 @@ struct OptimizeInstructions
     //
     if (auto* as = curr->ref->dynCast<RefAs>(); as && as->op == RefAsNonNull) {
       curr->ref = as->value;
-      curr->type = Type(curr->type.getHeapType(), NonNullable);
+      curr->type = curr->type.with(NonNullable);
     }
   }
 

test/lit/passes/optimize-instructions-exact.wast

@@ -1,6 +1,6 @@
 ;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
 
-;; RUN: wasm-opt %s -all --disable-custom-descriptors --optimize-instructions -S -o - | filecheck %s
+;; RUN: wasm-opt %s -all --optimize-instructions -S -o - | filecheck %s
 
 (module
  ;; CHECK:      (type $foo (sub (struct)))
@@ -63,4 +63,19 @@
    )
   )
  )
+
+ ;; CHECK:      (func $combine-non-null (type $3) (param $foo (ref null $foo)) (result (ref (exact $foo)))
+ ;; CHECK-NEXT:  (ref.cast (ref (exact $foo))
+ ;; CHECK-NEXT:   (local.get $foo)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $combine-non-null (param $foo (ref null $foo)) (result (ref (exact $foo)))
+  ;; We should not lose the exactness of the cast when we combine the
+  ;; ref.as_non_null into it.
+  (ref.cast (ref null (exact $foo))
+   (ref.as_non_null
+    (local.get $foo)
+   )
+  )
+ )
 )