[llvm] [SLP] Allow targets to add cost for nonstandard conditions (PR #95328)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

Add implementation of hasScalarizationOverhead and test.

Rebased on top of https://github.com/llvm/llvm-project/pull/91016 in order to facilitate the test



https://github.com/llvm/llvm-project/pull/95328