[llvm] [WIP][SLP] Allow external uses to exist as both an extract and a rematerialization during the pass (PR #211680)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Refactored most of the logic into an inner class of BoUpSLP. Still running tests (passes the LIT tests).

https://github.com/llvm/llvm-project/pull/211680