This is a simple move of CSA::AllocateRegExpResult to
RegExpBuiltinsAssembler. There's no reason for this method to be in CSA,
and this way we save a bit of binary size since code in builtins-*-gen
is not shipped.

Bug: v8:6741
Change-Id: I89507a0bfa4e0e922d92b9fcd0604ce86efea293
Reviewed-on: https://chromium-review.googlesource.com/626078Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47532}