[mlir][Transforms][NFC] Remove SplitBlockRewrite (#82777)

When splitting a block during a dialect conversion, a
`SplitBlockRewrite` object is stored in the dialect conversion state.
This commit removes `SplitBlockRewrite`. Instead, a combination of
`CreateBlockRewrite` and multiple `MoveOperationRewrite` is used.

This change simplifies the internal state of the dialect conversion and
is also needed to properly support listeners.

`RewriteBase::splitBlock` is now no longer virtual. All necessary
information for committing/rolling back a split block rewrite can be
deduced from `Listener::notifyBlockInserted` and
`Listener::notifyOperationInserted` (which is also called when moving an
operation).

Author: matthias-springer

mlir/include/mlir/IR/PatternMatch.h

@@ -579,7 +579,7 @@ class RewriterBase : public OpBuilder {
 
   /// Split the operations starting at "before" (inclusive) out of the given
   /// block into a new block, and return it.
-  virtual Block *splitBlock(Block *block, Block::iterator before);
+  Block *splitBlock(Block *block, Block::iterator before);
 
   /// Unlink this operation from its current block and insert it right before
   /// `existingOp` which may be in the same or another block in the same

mlir/include/mlir/Transforms/DialectConversion.h

@@ -741,9 +741,6 @@ class ConversionPatternRewriter final : public PatternRewriter {
   /// implemented for dialect conversion.
   void eraseBlock(Block *block) override;
 
-  /// PatternRewriter hook for splitting a block into two parts.
-  Block *splitBlock(Block *block, Block::iterator before) override;
-
   /// PatternRewriter hook for inlining the ops of a block into another block.
   void inlineBlockBefore(Block *source, Block *dest, Block::iterator before,
                          ValueRange argValues = std::nullopt) override;

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -192,7 +192,6 @@ class IRRewrite {
     EraseBlock,
     InlineBlock,
     MoveBlock,
-    SplitBlock,
     BlockTypeConversion,
     ReplaceBlockArg,
     // Operation rewrites
@@ -400,30 +399,6 @@ class MoveBlockRewrite : public BlockRewrite {
   Block *insertBeforeBlock;
 };
 
-/// Splitting of a block. This rewrite is immediately reflected in the IR.
-class SplitBlockRewrite : public BlockRewrite {
-public:
-  SplitBlockRewrite(ConversionPatternRewriterImpl &rewriterImpl, Block *block,
-                    Block *originalBlock)
-      : BlockRewrite(Kind::SplitBlock, rewriterImpl, block),
-        originalBlock(originalBlock) {}
-
-  static bool classof(const IRRewrite *rewrite) {
-    return rewrite->getKind() == Kind::SplitBlock;
-  }
-
-  void rollback() override {
-    // Merge back the block that was split out.
-    originalBlock->getOperations().splice(originalBlock->end(),
-                                          block->getOperations());
-    eraseBlock(block);
-  }
-
-private:
-  // The original block from which this block was split.
-  Block *originalBlock;
-};
-
 /// This structure contains the information pertaining to an argument that has
 /// been converted.
 struct ConvertedArgInfo {
@@ -883,9 +858,6 @@ struct ConversionPatternRewriterImpl : public RewriterBase::Listener {
   void notifyBlockInserted(Block *block, Region *previous,
                            Region::iterator previousIt) override;
 
-  /// Notifies that a block was split.
-  void notifySplitBlock(Block *block, Block *continuation);
-
   /// Notifies that a block is being inlined into another block.
   void notifyBlockBeingInlined(Block *block, Block *srcBlock,
                                Block::iterator before);
@@ -1522,11 +1494,6 @@ void ConversionPatternRewriterImpl::notifyBlockInserted(
   appendRewrite<MoveBlockRewrite>(block, previous, prevBlock);
 }
 
-void ConversionPatternRewriterImpl::notifySplitBlock(Block *block,
-                                                     Block *continuation) {
-  appendRewrite<SplitBlockRewrite>(continuation, block);
-}
-
 void ConversionPatternRewriterImpl::notifyBlockBeingInlined(
     Block *block, Block *srcBlock, Block::iterator before) {
   appendRewrite<InlineBlockRewrite>(block, srcBlock, before);
@@ -1665,15 +1632,6 @@ ConversionPatternRewriter::getRemappedValues(ValueRange keys,
                            results);
 }
 
-Block *ConversionPatternRewriter::splitBlock(Block *block,
-                                             Block::iterator before) {
-  assert(!impl->wasOpReplaced(block->getParentOp()) &&
-         "attempting to split a block within a replaced/erased op");
-  auto *continuation = block->splitBlock(before);
-  impl->notifySplitBlock(block, continuation);
-  return continuation;
-}
-
 void ConversionPatternRewriter::inlineBlockBefore(Block *source, Block *dest,
                                                   Block::iterator before,
                                                   ValueRange argValues) {