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

Port 34a1a76d

Original commit message:
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, mbrandy@us.ibm.com

BUG=

Review URL: https://codereview.chromium.org/1019003002

Cr-Commit-Position: refs/heads/master@{#27365}