[mlir][Linalg] Add folding of linalg.copy that are in fact identities.

mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp

 /// 2) The body contains just a yield operation with the yielded values being
 ///    the arguments corresponding to the operands.
 struct RemoveIdentityLinalgOps : public RewritePattern {
   RemoveIdentityLinalgOps(PatternBenefit benefit = 1)
       : RewritePattern(benefit, MatchAnyOpTypeTag()) {}
 
   LogicalResult matchAndRewrite(Operation *op,
                                 PatternRewriter &rewriter) const override {
+    if (auto copyOp = dyn_cast<CopyOp>(op)) {
+      assert(copyOp.hasBufferSemantics());
+      if (copyOp.input() == copyOp.output() &&
+          copyOp.inputPermutation() == copyOp.outputPermutation()) {
+        rewriter.eraseOp(op);
+        return success();
+      }
+    }
+
     if (!isa<GenericOp, IndexedGenericOp>(op))
       return failure();
     LinalgOp genericOp = cast<LinalgOp>(op);
     if (!genericOp.hasTensorSemantics())
       return failure();
     // Check all indexing maps are identity.
     if (llvm::any_of(genericOp.getIndexingMaps(),
                      [](AffineMap map) { return !map.isIdentity(); }))

mlir/test/Dialect/Linalg/canonicalize.mlir

func @propogate_casts(%arg0 : tensor<?x?xf32>, %arg1 : f32, %arg2 : index,
   return %4 : tensor<?x?xf32>
 }
 // CHECK-LABEL: func @propogate_casts
 //       CHECK:   %[[INIT:.+]] = linalg.init_tensor [21, 42]
 //       CHECK:   %[[FILL:.+]] = linalg.fill(%[[INIT]], %{{.+}})
 //       CHECK:   %[[INSERTED:.+]] = subtensor_insert %{{.+}} into %[[FILL]]
 //       CHECK:   %[[RESULT:.+]] = tensor.cast %[[INSERTED]]
 //       CHECK:   return %[[RESULT]]
+
+// -----
+
+// CHECK-LABEL: @self_copy
+func @self_copy(%arg0 : memref<2x3x?x4xf32>) {
+
+//   CHECK-NOT: linalg.copy
+  linalg.copy(%arg0, %arg0): memref<2x3x?x4xf32>, memref<2x3x?x4xf32>
+
+//   CHECK: return
+  return
+}