[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]

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bababuck wrote:

There is a slight costing change that is part of this MR. Prior to the MR, we only chose re-materialization if it was more that `TTI::TCC_Basic` cheaper than extracting or where there are non-power of two number of extracts. This is done because:
```
         // Keep original scalar if number of externally used instructions in
          // the same entry is not power of 2. It may help to do some extra
          // vectorization for now.
```
However, with this MR this isn't necessary, so we now choose to rematerialize if it is the same cost or cheaper.


See [here](https://github.com/llvm/llvm-project/pull/211680/changes#diff-e3b933e8c46156c0eaf7cbb67866b712f69a8484bc941d10acec9d4d17b9f061R20305-R20323), and this comment [here](https://github.com/llvm/llvm-project/pull/211680/changes#r3641839924). 

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