[PowerPC] dyn_cast should be dyn_cast_or_null in MASSV pass

It is possible that we have different constants in different slots
of second vector double (float) of pow function. So, in this case
Exp->getSplatValue() will return nullptr. Here, I handle it properly.

Reviewed By: steven.zhang, PowerPC

Differential Revision: https://reviews.llvm.org/D91729