[SVE] Fix incorrect usage of getNumElements() in InstCombineCalls

[ctetreau]

Summary

Remove incorrect usage of getNumElements() from visitCallInst(). The
number of elements was being used to construct a DemandedElts bitfield.
This operation does not make sense for scalable vectors. Cast to
FixedVectorType

Identified by test case Clang :: CodeGen/aarch64-sve-intrinsics/acle_sve_mla.c

[ctetreau]

This test case also hits the call to getNumElements() at IR/Function.cpp:1184. However, there is an open diff D79416 for this call site so I've left it unresolved in this patch.

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

if (auto *MI = dyn_cast<AnyMemIntrinsic>(II)) {
     } else if (auto *MSI = dyn_cast<AnyMemSetInst>(MI)) {
       if (Instruction *I = SimplifyAnyMemSet(MSI))
         return I;
     }
 
     if (Changed) return II;
   }
 
-  // For vector result intrinsics, use the generic demanded vector support.
-  if (auto *IIVTy = dyn_cast<VectorType>(II->getType())) {
-    auto VWidth = IIVTy->getNumElements();
+  // For fixed width vector result intrinsics, use the generic demanded vector
+  // support.
+  if (auto *IIFVTy = dyn_cast<FixedVectorType>(II->getType())) {
+    auto VWidth = IIFVTy->getNumElements();
     APInt UndefElts(VWidth, 0);
     APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
     if (Value *V = SimplifyDemandedVectorElts(II, AllOnesEltMask, UndefElts)) {
       if (V != II)
         return replaceInstUsesWith(*II, V);
       return II;
     }
   }