Apply case-insensitive comparisons not only for the initial character,
but for the entire prefix. This avoids degenerate behavior for patterns
like /aaaa|AAAA|AAAA/i (i.e. generate a single 4-char prefix instead of
four 1-char prefixes).

Bug: v8:12472
Change-Id: Ib2b49fe73ca846a1b7ec90056cc64bdf5cf33026
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3398114Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78668}