[mlir][Vector] Add `vector.to_elements` op (#141457)
This PR introduces the `vector.to_elements` op, which decomposes a
vector into its scalar elements. This operation is symmetrical to the
existing `vector.from_elements`.
Examples:
```
// Decompose a 0-D vector.
%0 = vector.to_elements %v0 : vector<f32>
// %0 = %v0[0]
// Decompose a 1-D vector.
%0:2 = vector.to_elements %v1 : vector<2xf32>
// %0#0 = %v1[0]
// %0#1 = %v1[1]
// Decompose a 2-D.
%0:6 = vector.to_elements %v2 : vector<2x3xf32>
// %0#0 = %v2[0, 0]
// %0#1 = %v2[0, 1]
// %0#2 = %v2[0, 2]
// %0#3 = %v2[1, 0]
// %0#4 = %v2[1, 1]
// %0#5 = %v2[1, 2]
```
This op is aimed at reducing code size when modeling "structured" vector
extractions and simplifying canonicalizations of large sequences of
`vector.extract` and `vector.insert` ops into `vector.shuffle` and other
sophisticated ops that can re-arrange vector elements.
