[IR] Constant::getAggregateElement - early-out for ScalableVectorType

We can't call getNumElements() for ScalableVectorType types - just bail for now, although ConstantAggregateZero/UndefValue could return a reasonable value.

Fixes crash shown in OSS-Fuzz #25272 https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=25272