[turbofan] Eliminate redundant ChangeTaggedSignedToInt32.

Apply the same strength reduction to the ChangeTaggedSignedToInt32
operator that we already have for ChangeTaggedToInt32.

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2428493002
Cr-Commit-Position: refs/heads/master@{#40352}

src/compiler/simplified-operator-reducer.cc

@@ -89,6 +89,7 @@ Reduction SimplifiedOperatorReducer::Reduce(Node* node) {
       }
       break;
     }
+    case IrOpcode::kChangeTaggedSignedToInt32:
     case IrOpcode::kChangeTaggedToInt32: {
       NumberMatcher m(node->InputAt(0));
       if (m.HasValue()) return ReplaceInt32(DoubleToInt32(m.Value()));