[mlir][vector] Remove usage of shapecast to remove unit dim

Instead of using shape_cast op in the pattern removing leading unit
dimensions we use extract/broadcast ops. This is part of the effort to
restrict ShapeCastOp fuirther in the future and only allow them to
convert to or from 1D vector.

This also adds extra canonicalization to fill the gaps in simplifying
broadcast/extract ops.

Differential Revision: https://reviews.llvm.org/D114205