[SVE] Fix incorrect assert in getDoubleElementsVectorType()

[david-arm]

Summary

In VectorType::getDoubleElementsVectorType we were asserting that
we could fit double the number of elements in an integer, however
the code was incorrectly using getVectorNumElements() for scalable
vectors. I've changed the code to use the element count instead.

[efriedma]

Do we have test coverage here?

[david-arm]

Hi @efriedma, I discovered this assert was generating warnings (due to getVectorNumElements call) whilst running some SVE intrinsics tests, i.e.

llvm-lit -a ../llvm/test/CodeGen/AArch64/sve-intrinsics-*.ll

so we do have existing test coverage. I found it difficult to know how to test this because I haven't really modified the code path.

[ctetreau]

I fixed this already in D78831

[david-arm]

This fix has been committed already by https://reviews.llvm.org/D78831

llvm/include/llvm/IR/DerivedTypes.h

assert ((EltCnt.Min & 1) == 0 &&
             "Cannot halve vector with odd number of elements.");
     return VectorType::get(VTy->getElementType(), EltCnt/2);
   }
 
   /// This static method returns a VectorType with twice as many elements as the
   /// input type and the same element type.
   static VectorType *getDoubleElementsVectorType(VectorType *VTy) {
     auto EltCnt = VTy->getElementCount();
-    assert((VTy->getNumElements() * 2ull) <= UINT_MAX &&
-           "Too many elements in vector");
+    assert((EltCnt.Min * 2ull) <= UINT_MAX && "Too many elements in vector");
     return VectorType::get(VTy->getElementType(), EltCnt*2);
   }
 
   /// Return true if the specified type is valid as a element type.
   static bool isValidElementType(Type *ElemTy);
 
   /// Return an ElementCount instance to represent the (possibly scalable)
   /// number of elements in the vector.