[llvm] [RISCV] Early exit if the type legalization cost is not valid for getIntrinsicInstrCost (PR #154256)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon commented:

> The motivation of this change is that a crash was encountered when the code calls TLI->getTypeToPromoteTo(ISD::FSQRT, FsqrtType) with type f16/bf16, but f16/bf16 wasn't promotable if zvfhmin/zvfbfmin are not enabled. This leads to an assertion failure.

Isn't the right fix something like the following?

```cpp
        if (!ST->hasVInstructionsF16Minimal())
          return InstructionCost::getInvalid();
```

```cpp
       if (!ST->hasVInstructionsBF16Minimal())
          return InstructionCost::getInvalid();
```

https://github.com/llvm/llvm-project/pull/154256