[llvm] [MIRParser] Reject a one-element fixed-length vector type (PR #213290)

[Matt Arsenault via llvm-commits]

================
@@ -1965,8 +1965,11 @@ static bool verifyScalarSize(uint64_t Size) {
   return Size != 0 && isUInt<16>(Size);
 }
 
-static bool verifyVectorElementCount(uint64_t NumElts) {
-  return NumElts != 0 && isUInt<16>(NumElts);
+static bool verifyVectorElementCount(uint64_t NumElts, bool HasVScale) {
+  // A fixed-length vector needs at least two elements: LLT::vector rejects a
+  // one-element ElementCount, and producers map such a type to the element
+  // type itself through LLT::scalarOrVector.
----------------
arsenm wrote:

```suggestion
  // A fixed-length vector needs at least two elements.
```

https://github.com/llvm/llvm-project/pull/213290