The vertex params are the 'start' vertex index and the number of
vertices in the draw call. For indexed draws, they sometimes mean we
need to compute the index range. In other cases, we don't. Defer
computing the index range as long as possible so we don't end up
computing the index range when we don't have to.

BUG=angleproject:2229

Change-Id: I8e125416ef6767787a14509b322efa8dcd8e4a34
Reviewed-on: https://chromium-review.googlesource.com/764676
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>