[llvm] [RISCV] Fix a bug in partial.reduce lowering for zvqdotq .vx forms (PR #142185)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@preames @topperc I've landed 4a7b53f04021d957db65248deafbfd71e333bc54 to fix a warning from this PR.

Now, your PR removed the last use of `ArgVT` aside from the one in `assert`.  Should we remove the assert?  Or should we keep it?  Thanks!

```
  MVT ArgVT = A.getSimpleValueType();
  assert(ArgVT == B.getSimpleValueType() &&
         ArgVT.getVectorElementType() == MVT::i8);
```


https://github.com/llvm/llvm-project/pull/142185