[llvm] [RISCV] Replace whitelist with elementsDependOnVL in RISCVVLOptimizer. NFC (PR #181619)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> # 🐧 Linux x64 Test Results
> * 169682 tests passed
> * 3059 tests skipped
> * 1 test failed
> 
> ## Failed Tests
> (click on a test name to see its output)
> 
> ### LLVM
> LLVM.CodeGen/RISCV/rvv/sifive_sf_vtdiscard.ll
> If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

This buildbot failure is probably irrelevant to this PR (since you didn't include #181601) -- it's caused by trying to access the def operand of an instruction that produce no result, and with proper `ElementsDependOn` configuration this won't be a problem. However, should we add a simple assertion in `isSupportedInstr` to bail out when an instruction produces no result but still opt-in to do VL reduction (i.e. `ElementsDependOn` set to EltDepsNone)?

https://github.com/llvm/llvm-project/pull/181619