[SLPVectorizer] Don't dereference a dyn_cast result. NFCI.

The static analyzer is warning about potential null dereferences of dyn_cast<> results - in these cases we can safely use cast<> directly as we know that these cases should all be the correct type, which is why its working atm and anyway cast<> will assert if they aren't.

llvm-svn: 371973