[SVE] Remove getNumElements() calls in visitGetElementPtrInst

[david-arm]

Summary

Replace calls to getNumElements() with getElementCount() in order
to avoid warnings for scalable vectors. The warnings were discovered
by this existing test:

test/CodeGen/AArch64/sve-gep.ll

llvm/lib/IR/Verifier.cpp

void Verifier::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   Assert(ElTy, "Invalid indices for GEP pointer type!", &GEP);
 
   Assert(GEP.getType()->isPtrOrPtrVectorTy() &&
              GEP.getResultElementType() == ElTy,
          "GEP is not of right type for indices!", &GEP, ElTy);
 
   if (auto *GEPVTy = dyn_cast<VectorType>(GEP.getType())) {
     // Additional checks for vector GEPs.
-    unsigned GEPWidth = GEPVTy->getNumElements();
+    ElementCount GEPWidth = GEPVTy->getElementCount();
     if (GEP.getPointerOperandType()->isVectorTy())
       Assert(
           GEPWidth ==
-              cast<VectorType>(GEP.getPointerOperandType())->getNumElements(),
+              cast<VectorType>(GEP.getPointerOperandType())->getElementCount(),
           "Vector GEP result width doesn't match operand's", &GEP);
     for (Value *Idx : Idxs) {
       Type *IndexTy = Idx->getType();
       if (auto *IndexVTy = dyn_cast<VectorType>(IndexTy)) {
-        unsigned IndexWidth = IndexVTy->getNumElements();
+        ElementCount IndexWidth = IndexVTy->getElementCount();
         Assert(IndexWidth == GEPWidth, "Invalid GEP index vector width", &GEP);
       }
       Assert(IndexTy->isIntOrIntVectorTy(),
              "All GEP indices should be of integer type");
     }
   }
 
   if (auto *PTy = dyn_cast<PointerType>(GEP.getType())) {