[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

 //      CHECK: #[[MAP:.+]] = affine_map<()[s0] -> (s0 * 28)>
 //      CHECK: func @dim_reshape_collapse
 // CHECK-SAME:   %[[ARG0:[a-zA-Z0-9_]+]]: tensor<2x3x5x4x?x7xf32>
 //  CHECK-DAG:   %[[C4:.+]] = constant 4 : index
 //  CHECK-DAG:   %[[C5:.+]] = constant 5 : index
 //      CHECK:   %[[D0:.+]] = dim %[[ARG0]], %[[C4]]
 //      CHECK:   %[[D1:.+]] = affine.apply #[[MAP]]()[%[[D0]]]
 //      CHECK:   return %[[C5]], %[[D1]]
+
+// -----
+
+// 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
+}