use [BUG] instead of [FIXME] in the sanity check fail debug msg

Author: GYT

llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

@@ -198,7 +198,7 @@ bool LoopVectorizeHints::allowVectorization(
     } else {
       // This should be unreachable unless there is a bug.
       LLVM_DEBUG(
-          dbgs() << "LV: [FIXME] Not vectorizing: loop vect disabled for "
+          dbgs() << "LV: [BUG] Not vectorizing: loop vect disabled for "
                     "an unknown reason!\n");
       ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedUnknown",
                                         TheLoop->getStartLoc(),