Drive-by-fix: Move IC::GetRootConstructor to Map::GetConstructorFunction,
so we can use that in the ICs, Crankshaft and Turbofan.

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

Review URL: https://codereview.chromium.org/1416493007

Cr-Commit-Position: refs/heads/master@{#31577}