[llvm] [WebAssembly] Implement getInterleavedMemoryOpCost (PR #146864)

[Thomas Lively via llvm-commits]

tlively wrote:

> > Is there a simple before-and-after example showing the effect of this change on codegen?
> 
> Are we still allowed to precommit tests without review? I could add the cost test and run it through llc?

I don't know what the policy is, but this would be helpful.

https://github.com/llvm/llvm-project/pull/146864