Add a clarifying comment about branching on poison

I recently got this wrong (again), and I'm sure I'm not the only one.  Put a comment in the logical place someone would look to "fix" the obvious "missed optimization" which arrises based on the common misunderstanding.  Hopefully, this will save others time.  :)

llvm-svn: 363318

llvm/lib/Analysis/ValueTracking.cpp

@@ -4388,6 +4388,10 @@ const Value *llvm::getGuaranteedNonFullPoisonOp(const Instruction *I) {
       return I->getOperand(1);
 
     default:
+      // Note: It's really tempting to think that a conditional branch or
+      // switch should be listed here, but that's incorrect.  It's not
+      // branching off of poison which is UB, it is executing a side effecting
+      // instruction which follows the branch.  
       return nullptr;
   }
 }