[mlir][scf] Fix a div-by-zero bug when step of scf.for is zero

Author: CoTinker

mlir/lib/Dialect/SCF/Transforms/LoopPipelining.cpp

@@ -119,6 +119,10 @@ bool LoopPipelinerInternal::initializeLoopInfo(
     int64_t ubImm = upperBoundCst.value();
     int64_t lbImm = lowerBoundCst.value();
     int64_t stepImm = stepCst.value();
+    if (stepImm <= 0) {
+      LDBG("--invalid loop step -> BAIL");
+      return false;
+    }
     int64_t numIteration = llvm::divideCeilSigned(ubImm - lbImm, stepImm);
     if (numIteration > maxStage) {
       dynamicLoop = false;