This CL changes Construct, ConstructVarargs, ConstructForwardVarargs and
ConstructFunctionForwardVarargs to preserve the kRootRegister (ebx).

Bug: v8:6666
Change-Id: Ia2c328e419506c029cda76fc210bbd45d9eb5952
Reviewed-on: https://chromium-review.googlesource.com/1233654Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56111}