[llvm] [RISCV] Remove fixed vector constraint on masked interleave costing (PR #150624)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

> > @preames [#151665 (comment)](https://github.com/llvm/llvm-project/pull/151665#discussion_r2251016129) @lukel97 and me plan to enable both fixed- and scalable-width masked interleave access support in #151665. Let me know if you think that's not appropriate. :)
> 
> Philip's out of office this week, I think you can just go ahead and land #151665 and add him as co-author if you want

I see, thanks.

https://github.com/llvm/llvm-project/pull/150624