[SVE][CodeGen] Replace use of TypeSize comparison operator in CreateStackTemporary

[david-arm]

Summary

We were previously relying upon the TypeSize comparison operators to
obtain the maximum size of two types, however use of such operators is
being deprecated in favour of making the caller aware that it could
be dealing with scalable vector types. I have changed the code to assert
that the two types have the same scalable property and thus we can
simply take the maximum of the known minimum sizes instead.

[c-rhodes]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

 SDValue SelectionDAG::CreateStackTemporary(EVT VT, unsigned minAlign) {
   Type *Ty = VT.getTypeForEVT(*getContext());
   Align StackAlign =
       std::max(getDataLayout().getPrefTypeAlign(Ty), Align(minAlign));
   return CreateStackTemporary(VT.getStoreSize(), StackAlign);
 }
 
 SDValue SelectionDAG::CreateStackTemporary(EVT VT1, EVT VT2) {
-  TypeSize Bytes = std::max(VT1.getStoreSize(), VT2.getStoreSize());
+  TypeSize VT1Size = VT1.getStoreSize();
+  TypeSize VT2Size = VT2.getStoreSize();
+  assert(VT1Size.isScalable() == VT2Size.isScalable() &&
+         "Don't know how to choose the maximum size when creating a stack "
+         "temporary");
+  TypeSize Bytes =
+      VT1Size.getKnownMinSize() > VT2Size.getKnownMinSize() ? VT1Size : VT2Size;
+
   Type *Ty1 = VT1.getTypeForEVT(*getContext());
   Type *Ty2 = VT2.getTypeForEVT(*getContext());
   const DataLayout &DL = getDataLayout();
   Align Align = std::max(DL.getPrefTypeAlign(Ty1), DL.getPrefTypeAlign(Ty2));
   return CreateStackTemporary(Bytes, Align);
 }
 
 SDValue SelectionDAG::FoldSetCC(EVT VT, SDValue N1, SDValue N2,