Use platform specific stubs for vector-based Load/KeyedLoad.

Use platform specific stubs for vector-based Load/KeyedLoad.

A hydrogen code stub is not the best approach because it builds a frame
and doesn't have the technology to discard roots at tail call exits.
Platform-specific stubs provide much better performance at this point.

R=verwaest@chromium.org

BUG=

Review URL: https://codereview.chromium.org/988653003

Cr-Commit-Position: refs/heads/master@{#27235}