This adds a single bottleneck that properly handles the copying of empty, COW
and FixedDoubleArray arrays under the control of flags. This is in preparation
of adding new CSA-based array builtins on Array.prototype.

Drive by: Fix SmiConstant handling when ENABLE_VERIFY_CSA is not active and 
make the use of constant detection/folding consistent in the CSA depending
on ParameterMode.

Change-Id: If1889ab8cbff1805286b7b4344c29ffbe7191b39
Reviewed-on: https://chromium-review.googlesource.com/715798
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48624}