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

[Luke Lau via llvm-commits]

https://github.com/lukel97 requested changes to this pull request.

Apropos of what @preames said, I think these a good chunk of these changes are actually covered by the cost model tests, they just need updated. Here's the list from the premerge CI:

```
  LLVM :: Analysis/CostModel/RISCV/abs.ll
  LLVM :: Analysis/CostModel/RISCV/active_lane_mask.ll
  LLVM :: Analysis/CostModel/RISCV/fp-min-max-abs.ll
  LLVM :: Analysis/CostModel/RISCV/fp-sqrt-pow.ll
  LLVM :: Analysis/CostModel/RISCV/fround.ll
  LLVM :: Analysis/CostModel/RISCV/int-bit-manip.ll
  LLVM :: Analysis/CostModel/RISCV/int-min-max.ll
  LLVM :: Analysis/CostModel/RISCV/int-sat-math.ll
  LLVM :: Analysis/CostModel/RISCV/rvv-cmp.ll
  LLVM :: Analysis/CostModel/RISCV/rvv-intrinsics.ll
  LLVM :: Analysis/CostModel/RISCV/rvv-select.ll
  LLVM :: Analysis/CostModel/RISCV/rvv-shuffle.ll
  LLVM :: Analysis/CostModel/RISCV/stepvector.ll
  LLVM :: Transforms/SLPVectorizer/RISCV/ctpop.ll
```

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