[BasicTTI] Return Invalid for scalable vectors reaching getScalarizationOverhead

If we would scalarize a fixed vector, we know we can't do so for a scalable one.  However, there's no need to crash, we can instead simply return a invalid cost which will work its way through the computation (since invalid is sticky), and the client should bail out.

Sorry for the lack of test here.  The particular codepath I saw this reached on was the result of another bug.

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -703,6 +703,8 @@ public:
                                            bool Insert, bool Extract) {
     /// FIXME: a bitfield is not a reasonable abstraction for talking about
     /// which elements are needed from a scalable vector
+    if (isa<ScalableVectorType>(InTy))
+      return InstructionCost::getInvalid();
     auto *Ty = cast<FixedVectorType>(InTy);
 
     assert(DemandedElts.getBitWidth() == Ty->getNumElements() &&
@@ -725,6 +727,8 @@ public:
   /// Helper wrapper for the DemandedElts variant of getScalarizationOverhead.
   InstructionCost getScalarizationOverhead(VectorType *InTy, bool Insert,
                                            bool Extract) {
+    if (isa<ScalableVectorType>(InTy))
+      return InstructionCost::getInvalid();
     auto *Ty = cast<FixedVectorType>(InTy);
 
     APInt DemandedElts = APInt::getAllOnes(Ty->getNumElements());