For small arrays, it's way faster to just move the elements instead of
doing the fairly complex and heavy-weight left-trimming. Crankshaft has
had this optimization for small arrays already; this CL more or less
ports this functionality to TurboFan, which yields a 4x speed-up when
using shift on small arrays (with up to 16 elements).

This should recover some of the regressions reported in the Node.js issues

  https://github.com/nodejs/node/issues/12657

and discovered for the syncthrough module using

  https://github.com/mcollina/syncthrough/blob/master/benchmarks/basic.js

as benchmark.

R=jarin@chromium.org
BUG=v8:6376

Review-Url: https://codereview.chromium.org/2874453002
Cr-Commit-Position: refs/heads/master@{#45216}