[llvm] [RISCV][TTI] Scale the cost of the sext/zext with LMUL (PR #86617)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

reverted since it is failing linux bots https://lab.llvm.org/buildbot/#/builders/5/builds/42101.
```
llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp:936:37: runtime error: index -1 out of bounds for type 'unsigned int[3]'
```

https://github.com/llvm/llvm-project/pull/86617