We no longer use the terminology "fast elements", so drop the "Fast"
from both NewFastSmiOrObjectElements and NewFastDoubleElements operator
names.

Bug: v8:6399, v8:6901
Tbr: jarin@chromium.org
Change-Id: Icc204623f2b459b0d0e172e26ddd73e29fe6c884
Reviewed-on: https://chromium-review.googlesource.com/707246Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48399}