Note that case-insensitive prefixes are already lowercase.

Change-Id: Idbccb386ed50cbd3b40c72604aa42f0570e50a89
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59030
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit 4606f89f43b2991908b904576bb1b936891115bc)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59050