[MachineValueType] Don't allow MVT::getVectorNumElements() to be called for scalable vectors.

Migrate the one caller that failed lit tests to use
MVT::getVectorMinNumElements directly.

Author: topperc

llvm/include/llvm/Support/MachineValueType.h

@@ -848,7 +848,11 @@ namespace llvm {
     }
 
     unsigned getVectorNumElements() const {
-      // TODO: Check that this isn't a scalable vector.
+      if (isScalableVector())
+        llvm::reportInvalidSizeRequest(
+            "Possible incorrect use of MVT::getVectorNumElements() for "
+            "scalable vector. Scalable flag may be dropped, use "
+            "MVT::getVectorElementCount() instead");
       return getVectorMinNumElements();
     }
 

llvm/lib/Support/LowLevelType.cpp

@@ -17,7 +17,7 @@ using namespace llvm;
 
 LLT::LLT(MVT VT) {
   if (VT.isVector()) {
-    bool asVector = VT.getVectorNumElements() > 1;
+    bool asVector = VT.getVectorMinNumElements() > 1;
     init(/*IsPointer=*/false, asVector, /*IsScalar=*/!asVector,
          VT.getVectorElementCount(), VT.getVectorElementType().getSizeInBits(),
          /*AddressSpace=*/0);