[compiler] Re-add NaN-folding optimization

This was previously removed in
https://chromium-review.googlesource.com/c/v8/v8/+/946129.

Given that test/mjsunit/compiler/regress-817225.js no longer
reproduces, and that the original CL removed only one occurrence of
this common pattern, it's not clear that it fixes anything.

Bug: v8:7519
Change-Id: I973a581e1e6cdea5ba2ff31364bd6701602fc8d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2637854
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72212}

src/compiler/machine-operator-reducer.cc

@@ -508,6 +508,12 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
     }
     case IrOpcode::kFloat64Add: {
       Float64BinopMatcher m(node);
+      if (m.right().IsNaN()) {  // x + NaN => NaN
+        return ReplaceFloat64(SilenceNaN(m.right().ResolvedValue()));
+      }
+      if (m.left().IsNaN()) {  // NaN + x => NaN
+        return ReplaceFloat64(SilenceNaN(m.left().ResolvedValue()));
+      }
       if (m.IsFoldable()) {  // K + K => K  (K stands for arbitrary constants)
         return ReplaceFloat64(m.left().ResolvedValue() +
                               m.right().ResolvedValue());