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

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. I should mention that this prevents some loops being vectorized in the llvm-test-suite. But this is probably what we want, since if we use LMUL=2 in the loop vectorizer, then we were costing certain operations as half of what they should be. I've attached the diff below
[upstream-scale-lmul.diff.txt](https://github.com/llvm/llvm-project/files/13977077/upstream-scale-lmul.diff.txt)


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