Remove a FIXME that will never be fixed since undef is being removed.

[nikic]

LGTM

llvm/lib/Transforms/Scalar/BDCE.cpp

for (Use &U : I.operands()) {
 
       if (!DB.isUseDead(&U))
         continue;
 
       LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
 
       clearAssumptionsOfUsers(&I, DB);
 
-      // FIXME: In theory we could substitute undef here instead of zero.
-      // This should be reconsidered once we settle on the semantics of
-      // undef, poison, etc.
+      // Substitute all uses with zero. In theory we could use `freeze poison`
+      // instead, but that seems unlikely to be profitable.
       U.set(ConstantInt::get(U->getType(), 0));
       ++NumSimplified;
       Changed = true;
     }
   }
 
   for (Instruction *&I : llvm::reverse(Worklist)) {
     salvageDebugInfo(*I);