[CodeGen] Fix warnings in getCopyToPartsVector

[david-arm]

Summary

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.

[efriedma]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

NumRegs =
         TLI.getVectorTypeBreakdown(*DAG.getContext(), ValueVT, IntermediateVT,
                                    NumIntermediates, RegisterVT);
   }
 
   assert(NumRegs == NumParts && "Part count doesn't match vector breakdown!");
   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) {
     if (SDValue Widened = widenVectorToPartType(DAG, Val, DL, BuiltVectorTy))
       Val = Widened;
 
     Val = DAG.getNode(ISD::BITCAST, DL, BuiltVectorTy, Val);
   }
 
   // Split the vector into intermediate operands.
   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));
     } else {
       Ops[i] = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, IntermediateVT, Val,
                            DAG.getVectorIdxConstant(i, DL));
     }
   }