[Turbofan] Only weaken types for Phi nodes.

We also need to fix the weakening to weaken unions with ranges in them.

BUG=

Review URL: https://codereview.chromium.org/1011103003

Cr-Commit-Position: refs/heads/master@{#27257}

src/compiler/typer.cc

@@ -365,9 +365,11 @@ class Typer::Visitor : public Reducer {
     if (NodeProperties::IsTyped(node)) {
       // Widen the bounds of a previously typed node.
       Bounds previous = NodeProperties::GetBounds(node);
+      if (node->opcode() == IrOpcode::kPhi) {
         // Speed up termination in the presence of range types:
         current.upper = Weaken(current.upper, previous.upper);
         current.lower = Weaken(current.lower, previous.lower);
+      }
 
       // Types should not get less precise.
       DCHECK(previous.lower->Is(current.lower));
@@ -1309,16 +1311,14 @@ Type* Typer::Visitor::Weaken(Type* current_type, Type* previous_type) {
     return current_type;
   }
 
-  Type* previous_number =
-      Type::Intersect(previous_type, typer_->integer, zone());
-  Type* current_number = Type::Intersect(current_type, typer_->integer, zone());
-  if (!current_number->IsRange() || !previous_number->IsRange()) {
+  Type::RangeType* previous =
+      Type::Intersect(previous_type, typer_->integer, zone())->GetRange();
+  Type::RangeType* current =
+      Type::Intersect(current_type, typer_->integer, zone())->GetRange();
+  if (current == nullptr || previous == nullptr) {
     return current_type;
   }
 
-  Type::RangeType* previous = previous_number->AsRange();
-  Type::RangeType* current = current_number->AsRange();
-
   double current_min = current->Min();
   double new_min = current_min;
   // Find the closest lower entry in the list of allowed