Avoid unnecessary is8Bit() check for each string in JSStringJoiner when m_isAll8Bit is true.
Pull request: https://github.com/WebKit/WebKit/pull/11982
Committed 262172@main (53d70e1e2b39): <https://commits.webkit.org/262172@main> Reviewed commits have been landed. Closing PR #11982 and removing active labels.
<rdar://problem/107281750>