[mlir][vector] Fold scalar vector.extract of non-splat n-D constants

Add a new pattern to fold `vector.extract` over n-D constants that extract scalars.
The previous code handled ND splat constants only. The new pattern is conservative and does handle sub-vector constants.

This is to aid the `arith::EmulateWideInt` pass which emits a lot of 2-element vector constants.

Reviewed By: Mogball, dcaballe

Differential Revision: https://reviews.llvm.org/D133742