[ValueTracking] Replace an always false condition with an assert. NFC

The one caller of this function already checked that V isn't a
Constant.

Alternatively, we could remove the check from the caller if reviewers
prefer.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D143677

Author: topperc

llvm/lib/Analysis/ValueTracking.cpp

@@ -2302,8 +2302,7 @@ static bool isGEPKnownNonNull(const GEPOperator *GEP, unsigned Depth,
 static bool isKnownNonNullFromDominatingCondition(const Value *V,
                                                   const Instruction *CtxI,
                                                   const DominatorTree *DT) {
-  if (isa<Constant>(V))
-    return false;
+  assert(!isa<Constant>(V) && "Called for constant?");
 
   if (!CtxI || !DT)
     return false;