We can remove a lot of native code and rely on CallOrConstructVarargs
to do the stack manipulation for us.

This will also take advantage of the fast-path for double arrays in
CallOrConstructDoubleVarargs.

We can also remove Runtime_SpreadIterableFixed because it isn't used
anymore. We just call directly into spread_iterable from CSA.

Bug: v8:6488, chromium:704966
Change-Id: I81a18281f062619851134fff7ce88471566ee3b5
Reviewed-on: https://chromium-review.googlesource.com/535615Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46038}