Implement relaxed_swizzle opcode from relaxed SIMD proposal
<rdar://problem/110495643>
Pull request: https://github.com/WebKit/WebKit/pull/14797
Committed 265075@main (e1ec14877b7d): <https://commits.webkit.org/265075@main> Reviewed commits have been landed. Closing PR #14797 and removing active labels.