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

[Mel Chen via llvm-commits]

Mel-Chen wrote:

@preames 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. :)

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