Remove an unused variable in release build.

Author: hokein

llvm/lib/Analysis/ValueTracking.cpp

@@ -4279,9 +4279,8 @@ static KnownFPClass computeKnownFPClassFromContext(const Value *V,
     if (!AssumeVH)
       continue;
     CallInst *I = cast<CallInst>(AssumeVH);
-    const Function *F = I->getFunction();
 
-    assert(F == Q.CxtI->getParent()->getParent() &&
+    assert(I->getFunction() == Q.CxtI->getParent()->getParent() &&
            "Got assumption for the wrong function!");
     assert(I->getCalledFunction()->getIntrinsicID() == Intrinsic::assume &&
            "must be an assume intrinsic");