[VPlanSLP] Don't dereference a cast_or_null<VPInstruction> result. NFCI.

The static analyzer is warning about a potential null dereference of the cast_or_null result, I've split the cast_or_null check from the ->getUnderlyingInstr() call to avoid this, but it appears that we weren't seeing any null pointers in the dumped bundles in the first place.

llvm-svn: 371975