[llvm] [SLP] Enable reordering for non-power-of-two vectors (PR #106638)

[Philip Reames via llvm-commits]

preames wrote:

When pulling this down through internal merge, I found a problem (crash).  I'm reducing now, and will either post a fix short if this turns out to be easy, or revert.  

https://github.com/llvm/llvm-project/pull/106638