[llvm] [SLP]Support copyable GEP/zext main ops and relax gather pointer compatibility (PR #221599)

[Ryan Buchner via llvm-commits]

https://github.com/bababuck approved this pull request.

LGTM, I'm still going to try to hit that case we discussed but that will just be perf related (if there is an issue there) so I don't want to block the rest of this for that.

https://github.com/llvm/llvm-project/pull/221599