[LV] Fix bug in setVectorizedCallDecision (#175742)

There is a bug in this logic:

```
   InstructionCost Cost = ScalarCost;
   InstWidening Decision = CM_Scalarize;

   if (VectorCost <= Cost) {
     Cost = VectorCost;
     Decision = CM_VectorCall;
   }

   if (IntrinsicCost <= Cost) {
     Cost = IntrinsicCost;
     Decision = CM_IntrinsicCall;
   }
```

because it assumes that the comparisons behave sensibly in the face of
invalid costs. Unfortunately, PR #174835 exposes an issue when
attempting to vectorise the new test
uadd_with_overflow_i32 for AArch64 targets. Specifically, there are
situations where all costs are invalid (e.g. VF=vscale x 1), but some
costs are more invalid than others. For example, when querying the
intrinsic cost via the TTI hook we get an invalid cost with a non-zero
value, whereas the vector cost is invalid with a zero value. That leads
to us erroneously choosing CM_VectorCall as the call widening decision,
despite the lack of a vector math variant. Inevitably this causes
crashes because we create a VPCallWidenRecipe without a variant
function.

Fix this by only performing comparisons if the costs are valid. It now
leads to us choosing CM_Scalarize more often, but it's a toin coss
anyway between CM_Scalarize and CM_IntrinsicCall when both strategies
are invalid. Potentially we could also create a new strategy called
CM_Invalid, and avoid the creation of VPlans entirely.