[compiler] Restore two optimizations of Float64Pow

These appear to be consistent with the pow implementation in ieee754.cc.

Bug: v8:11371
Change-Id: I1b695facb5ba7dc1a7bd28914bdb535966e080c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2710432
Auto-Submit: Georg Neis <neis@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72922}

src/compiler/machine-operator-reducer.cc

@@ -737,6 +737,13 @@ Reduction MachineOperatorReducer::Reduce(Node* node) {
                                                  m.right().ResolvedValue()));
       } else if (m.right().Is(0.0)) {  // x ** +-0.0 => 1.0
         return ReplaceFloat64(1.0);
+      } else if (m.right().Is(2.0)) {  // x ** 2.0 => x * x
+        node->ReplaceInput(1, m.left().node());
+        NodeProperties::ChangeOp(node, machine()->Float64Mul());
+        return Changed(node);
+      } else if (m.right().Is(0.5)) {
+        // x ** 0.5 => if x <= -Infinity then Infinity else sqrt(0.0 + x)
+        return Replace(Float64PowHalf(m.left().node()));
       }
       break;
     }