Optimizing global constants such as "NaN", "Infinity" and "undefined" is
best performed during graph building. Then the optimization and lowering
passes only need to deal with real loads in case of JSLoadGlobal.

R=mstarzinger@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1384953002

Cr-Commit-Position: refs/heads/master@{#31135}