[llvm] [SLP][TTI]Add support for strided loads. (PR #80310)

[Philip Reames via llvm-commits]

preames wrote:

> > Can I ask you to separate the TTI pieces to a separate review? I'd be comfortable LGTMing that with some pretty minimal changes, but can't reasonable review the SLP changes in any depth.
> 
> I can't, since don't know how to test TTI changes without SLP vectorizer support.

The change would be untested on it's own.  I'm explicitly saying I'm willing to approve that as an exception to usual policy.  

Partly because if we can get that in, I plan to write a patch which exercises at least the most basic cases through the instruction cost printer pass.  

https://github.com/llvm/llvm-project/pull/80310