[mlir][linalg] PadTensorOp vectorization: Avoid redundant FillOps

Do not generate FillOps when these would be entirely overwritten.

Differential Revision: https://reviews.llvm.org/D109741

mlir/lib/Dialect/Linalg/Transforms/Vectorization.cpp

@@ -751,6 +751,13 @@ struct GenericPadTensorOpVectorizationPattern
         padOp.getLoc(), vecType, padOp.source(), readIndices, padValue,
         readInBounds);
 
+    // If `dest` is a FillOp and the TransferWriteOp would overwrite the entire
+    // tensor, write directly to the FillOp's operand.
+    if (llvm::equal(vecShape, resultType.getShape())
+        && llvm::all_of(writeInBounds, [](bool b) { return b; }))
+      if (auto fill = dest.getDefiningOp<FillOp>())
+        dest = fill.output();
+
     // Generate TransferWriteOp.
     auto writeIndices = ofrToIndexValues(
         rewriter, padOp.getLoc(), padOp.getMixedLowPad());