[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:

Pulled out extra logic so that there is no longer any costing change, also removed support for extended loads, so now only allow deferral for `LoadInsts`. This simplifes the overall MR and reduces the number of test changes.

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