[mlir][Linalg] Add a pattern to decompose `linalg.generic` ops.

This patch adds a pattern to decompose a `linalg.generic` operations
that
- has only parallel iterator types
- has more than 2 statements (including the yield)

into multiple `linalg.generic` operation such that each operation has
a single statement and a yield.
The pattern added here just splits the matching `linalg.generic` into
two `linalg.generic`s, one containing the first statement, and the
other containing the remaining. The same pattern can be applied
repeatedly on the second op to ultimately fully decompose the generic
op.

Differential Revision: https://reviews.llvm.org/D129704