[CodeGen] Fix warnings in getCopyToPartsVector

Whilst trying to assemble the following test:

  clang/test/CodeGen/aarch64-sve-intrinsics/acle_sve_set2.c

I discovered we were hitting some warnings about possible invalid
calls to getVectorNumElements() in getCopyToPartsVector(). I've
tried to fix these by using ElementCount types where possible and
I've made the assumption that we don't support using a fixed width
vector to copy parts of a scalable vector, and vice versa. Looking
at how the copy is implemented I think that's the right thing for
now.

Differential Revision: https://reviews.llvm.org/D82744

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -745,12 +745,16 @@ static void getCopyToPartsVector(SelectionDAG &DAG, const SDLoc &DL,
   NumParts = NumRegs; // Silence a compiler warning.
   assert(RegisterVT == PartVT && "Part type doesn't match vector breakdown!");
 
-  unsigned IntermediateNumElts = IntermediateVT.isVector() ?
-    IntermediateVT.getVectorNumElements() : 1;
+  assert(IntermediateVT.isScalableVector() == ValueVT.isScalableVector() &&
+         "Mixing scalable and fixed vectors when copying in parts");
+
+  ElementCount DestEltCnt;
+
+  if (IntermediateVT.isVector())
+    DestEltCnt = IntermediateVT.getVectorElementCount() * NumIntermediates;
+  else
+    DestEltCnt = ElementCount(NumIntermediates, false);
 
-  // Convert the vector to the appropriate type if necessary.
-  auto DestEltCnt = ElementCount(NumIntermediates * IntermediateNumElts,
-                                 ValueVT.isScalableVector());
   EVT BuiltVectorTy = EVT::getVectorVT(
       *DAG.getContext(), IntermediateVT.getScalarType(), DestEltCnt);
   if (ValueVT != BuiltVectorTy) {
@@ -764,6 +768,9 @@ static void getCopyToPartsVector(SelectionDAG &DAG, const SDLoc &DL,
   SmallVector<SDValue, 8> Ops(NumIntermediates);
   for (unsigned i = 0; i != NumIntermediates; ++i) {
     if (IntermediateVT.isVector()) {
+      // This does something sensible for scalable vectors - see the
+      // definition of EXTRACT_SUBVECTOR for further details.
+      unsigned IntermediateNumElts = IntermediateVT.getVectorMinNumElements();
       Ops[i] =
           DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, IntermediateVT, Val,
                       DAG.getVectorIdxConstant(i * IntermediateNumElts, DL));