The `capture_ix` refers to all captures while `capture_count` only
refers to named captures. Clarified by renaming `capture_count` to
`named_capture_count` and removing the incorrect part of the DCHECK.

The `>= 1` part of the condition must still hold since named captures
can only refer to explicit capture groups, which start at index 1.

Tbr: petermarshall@chromium.org
Bug: chromium:1018592
Change-Id: If8a26f6661ba0483d585f74270b3b4a3853e2ca8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1886810Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64629}