When lowering a JSLoadNamed with an immutable own property of a
known constant JSObject, we can just constant fold the load during
native context specialization, similar to what Crankshaft does in
HOptimizedGraphBuilder::BuildLoadNamedField. This way we can also
constant-fold stuff like Math.LN2 and friends.

R=jarin@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2376033002
Cr-Commit-Position: refs/heads/master@{#39849}