[llvm] [RISCV] Fix missing scaling by LMUL in cost model (PR #73342)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Ping. @lukel97 I think the regression you pointed out to me should have disappeared after the latest changes.

Thanks. I also ran this on TSCV2 with `-O3 -march=rv64gv`, and there doesn't seem to be any changes between this and 50c298fd174f1a98fd20374a65d1faecba5649e1 

https://github.com/llvm/llvm-project/pull/73342