[NFC] Remove a FIXME fixed by an earlier patch.

[ymandel]

Summary

Commit 28bd7945eabdbde2b1fc071ab2f9b78e6e754a1a incidentally fixed the
associated FIXME, but didn't delete it.

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

 /// meaning depends on `Model`.
 static Value *mergeDistinctValues(QualType Type, Value *Val1,
                                   const Environment &Env1, Value *Val2,
                                   const Environment &Env2,
                                   Environment &MergedEnv,
                                   Environment::ValueModel &Model) {
   // Join distinct boolean values preserving information about the constraints
   // in the respective path conditions.
-  //
-  // FIXME: Does not work for backedges, since the two (or more) paths will not
-  // have mutually exclusive conditions.
   if (auto *Expr1 = dyn_cast<BoolValue>(Val1)) {
     auto *Expr2 = cast<BoolValue>(Val2);
     auto &MergedVal = MergedEnv.makeAtomicBoolValue();
     MergedEnv.addToFlowCondition(MergedEnv.makeOr(
         MergedEnv.makeAnd(Env1.getFlowConditionToken(),
                           MergedEnv.makeIff(MergedVal, *Expr1)),
         MergedEnv.makeAnd(Env2.getFlowConditionToken(),
                           MergedEnv.makeIff(MergedVal, *Expr2))));