[mlir][linalg] Clarify comments and remove outdated TODO (nfc) (#171695)

The TODO being removed was originally added at my request in:
  * #160246

Upon revisiting the issue, it becomes clear that implementing the TODO
would require supporting IR of the following form (note the static tile
size `8` and the corresponding dynamic dimension in the result type):

```mlir
%pack = linalg.pack %src
  padding_value(%c5 : i32)
  inner_dims_pos = [0, 1]
  inner_tiles = [8, 1]
  into %dest : tensor<?x?xi32> -> tensor<1x1x?x1xi32>
```

Allowing this would be invalid, as discussed here:
* https://discourse.llvm.org/t/tensor-ops-with-dynamic-sizes-which-behaviour-is-more-correct

Author: banach-space

mlir/include/mlir/Dialect/Linalg/IR/LinalgRelayoutOps.td

@@ -58,11 +58,14 @@ class Linalg_RelayoutOp<string mnemonic, list<Trait> traits = []> :
     /// tile factors.
     DenseMap<int64_t, OpFoldResult> getDimAndTileMapping();
 
-    // TODO: Return the folded result.
-    /// Return the tile sizes as OpFoldResult. Will return the Value
-    /// of the constant Op, not the constant Attribute.
-    /// E.g., for %size = arith.constant 1 : i32 will return %size,
-    /// not 1.
+    /// Return the tile sizes as OpFoldResult(s). Note, for Ops that simply
+    /// define constants (e.g. `arith.constant`), this method returns
+    /// Value/result of the Op (as opposed to the corresponding constant
+    /// Attribute). E.g., for:
+    ///   %size = arith.constant 1 : i32
+    /// it will return %size, not 1. This is intended - replacing an SSA value
+    /// with a constant attribute would require updating the corresponding dim
+    /// from dynamic to static.
     SmallVector<OpFoldResult> getMixedTiles();
 
     /// Return the tile sizes as `int64_t`. If a tile size is dynamic

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

@@ -1219,8 +1219,11 @@ LogicalResult DecomposeOuterUnitDimsPackOpPattern::matchAndRewrite(
   }
   shapeForEmptyOp.append(packOp.getMixedTiles());
 
-  // getMixedTiles() may contain Values pointing to constant ops, not the
-  // constant attributes. Replace them with a true OpFoldResult.
+  // getMixedTiles() may contain Values pointing to constant ops (as opposed to
+  // constant attributes with the corresponding value). Replace those with
+  // attributes. This is to match the behaviour in
+  // `getPackOpSourceOrPaddedSource`, which replaces constant SSA values with
+  // attributes.
   llvm::transform(shapeForEmptyOp, shapeForEmptyOp.begin(),
                   [&](OpFoldResult ofr) {
                     if (auto val = llvm::dyn_cast<Value>(ofr))