Fix crash in optimizeSelect with non-concrete arm types

src/passes/OptimizeInstructions.cpp

@@ -3415,6 +3415,12 @@ struct OptimizeInstructions
           if (canReorder(ifTrue, c)) {
             return builder.makeSequence(builder.makeDrop(c), ifTrue);
           }
+          // We generally skip optimizing unreachable selects, but an
+          // optimization on the children might have made them newly
+          // unreachable. We cannot create a scratch local in that case.
+          if (!ifTrue->type.isConcrete()) {
+            return nullptr;
+          }
           auto scratch = builder.addVar(getFunction(), ifTrue->type);
           return builder.makeBlock(
             {builder.makeLocalSet(scratch, ifTrue),

test/lit/passes/optimize-instructions-gc-tnh.wast

@@ -1056,4 +1056,39 @@
   (func $get-null (result (ref null none))
     (unreachable)
   )
+
+  ;; TNH:      (func $select-uninhabitable-arms (type $void)
+  ;; TNH-NEXT:  (drop
+  ;; TNH-NEXT:   (select
+  ;; TNH-NEXT:    (unreachable)
+  ;; TNH-NEXT:    (unreachable)
+  ;; TNH-NEXT:    (call $get-i32)
+  ;; TNH-NEXT:   )
+  ;; TNH-NEXT:  )
+  ;; TNH-NEXT: )
+  ;; NO_TNH:      (func $select-uninhabitable-arms (type $void)
+  ;; NO_TNH-NEXT:  (drop
+  ;; NO_TNH-NEXT:   (select
+  ;; NO_TNH-NEXT:    (unreachable)
+  ;; NO_TNH-NEXT:    (unreachable)
+  ;; NO_TNH-NEXT:    (call $get-i32)
+  ;; NO_TNH-NEXT:   )
+  ;; NO_TNH-NEXT:  )
+  ;; NO_TNH-NEXT: )
+  (func $select-uninhabitable-arms
+    (drop
+      ;; This select is not unreachable, so we will optimize it.
+      (select (result (ref nofunc))
+        ;; But the arms will be optimized to unreachable first. We should not
+        ;; crash on trying to create an unreachable scratch local here.
+        (ref.as_non_null
+          (ref.null nofunc)
+        )
+        (ref.as_non_null
+          (ref.null nofunc)
+        )
+        (call $get-i32)
+      )
+    )
+  )
 )