Optimizes BitVector::Iterator::Advance by using base::bits::CountTrailingZeros to
skip through bitvector. Also inlines Advance in the header. This reduces the
LiveRangeAnalysis phase of TurboFan/Prop by about 2-5% on Octane.

BUG=v8:9684

Change-Id: I3954d50d8ae9bd062a153e1fa2cb0abfd43d73eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1910948
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64920}