[llvm] [RISCV][TTI] Fix crash of non-built-in vector type cost quering. (PR #167258)

[Luke Lau via llvm-commits]

================
@@ -6239,3 +6239,13 @@ define void @legalization_crash() {
   fptoui <192 x float> undef to <192 x i1>
   ret void
 }
+
+; Test that BitInt cases won't crash at cost model.
----------------
lukel97 wrote:

Nit, maybe say that this is testing cases where one of the types needs to be split?

```suggestion
; Test that types that need to be split go through BasicTTIImpl
```

https://github.com/llvm/llvm-project/pull/167258