[X86] getMemoryOpCost - use dyn_cast_or_null<StoreInst>. NFCI.

Use instead of the isa_and_nonnull<StoreInst> and use the StoreInst::getPointerOperand wrapper instead of a hardcoded Instruction::getOperand.

Looks cleaner and avoids a spurious clang static analyzer null dereference warning.