[ValueTracking] Silence static analyzer dyn_cast<Operator> null dereference warnings. NFCI.

The static analyzer is warning about a potential null dereferences, but since the pointer is only used in a switch statement for Operator::getOpcode() (with an empty default) then its easiest just to wrap this in a null test as the dyn_cast might return null here.

llvm-svn: 372962