Use the `Base` type alias from
https://github.com/llvm/llvm-project/pull/158433.

See https://github.com/llvm/llvm-project/pull/147168 for more info.

This patch fixes:

  mlir/lib/Dialect/Vector/Transforms/LowerVectorToFromElementsToShuffleTree.cpp:42:20:
  error: unused variable 'kIndScale' [-Werror,-Wunused-const-variable]

This PR adds a new transformation that turns sequences of `vector.to_elements` and `vector.from_elements` into a binary tree of `vector.shuffle` operations.

(Related RFC:
https://discourse.llvm.org/t/rfc-adding-vector-to-elements-op-to-the-vector-dialect/86779).

Example:

```
  %0:4 = vector.to_elements %a : vector<4xf32>
  %1:4 = vector.to_elements %b : vector<4xf32>
  %2:4 = vector.to_elements %c : vector<4xf32>
  %3 = vector.from_elements %0#0, %0#1, %0#2, %0#3,
                            %1#0, %1#1, %1#2, %1#3,
                            %2#0, %2#1, %2#2, %2#3 : vector<12xf32>

==>

  %0 = vector.shuffle %a, %b [0, 1, 2, 3, 4, 5, 6, 7] : vector<4xf32>, vector<4xf32>
  %1 = vector.shuffle %c, %c [0, 1, 2, 3, -1, -1, -1, -1] : vector<4xf32>, vector<4xf32>
  %2 = vector.shuffle %0, %1 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] : vector<8xf32>, vector<8xf32>
```

The algorithm leverages the structured extraction/insertion information
of `vector.to_elements` and `vector.from_elements` operations and builds
a set of intervals to determine the vector length that should be used at
each level of the tree to combine the level inputs in pairs.

There are a few improvements that can be implemented in the future, such
as shuffle mask compression to avoid unnecessarily large vector lengths
with poison values, but I decided to keep things "simpler" and spend
more time documenting the different steps of the algorithm so that
people can follow along.