[mlir] Use Region::hasOneBlock (NFC) (#133879)

Author: CoTinker

mlir/lib/Dialect/SPIRV/IR/ControlFlowOps.cpp

@@ -308,7 +308,7 @@ LogicalResult LoopOp::verifyRegions() {
     return emitOpError(
         "should not have 'spirv.mlir.merge' op outside the merge block");
 
-  if (std::next(region.begin()) == region.end())
+  if (region.hasOneBlock())
     return emitOpError(
         "must have an entry block branching to the loop header block");
   // The first block is the entry block.
@@ -502,7 +502,7 @@ LogicalResult SelectionOp::verifyRegions() {
     return emitOpError(
         "should not have 'spirv.mlir.merge' op outside the merge block");
 
-  if (std::next(region.begin()) == region.end())
+  if (region.hasOneBlock())
     return emitOpError("must have a selection header block");
 
   return success();

mlir/lib/Transforms/Utils/RegionUtils.cpp

@@ -200,7 +200,7 @@ LogicalResult mlir::eraseUnreachableBlocks(RewriterBase &rewriter,
       continue;
 
     // If this is a single block region, just collect the nested regions.
-    if (std::next(region->begin()) == region->end()) {
+    if (region->hasOneBlock()) {
       for (Operation &op : region->front())
         for (Region &region : op.getRegions())
           worklist.push_back(&region);