Refactoring ideas for the stencil dialect

[gysit]

• use memref instead of the stencil types
• use an X in the stencil access notation to mark unused dimensions
• add a dimensionality attribute to the stencil apply to support stencils with different dimensionality
• handle cases where stencils for example load non overlapping bounding boxes from the same input array (having one stencil.load makes no sense in these examples). Introduce some sort of combine / split on the inputs
• introduce a new operation (some sort of nop) that can stop fusion between two stencils to control the stencil inlining

[tobiasgrosser]

I like in particular the last idea.

[gysit]

yes that could be an easy way to control optimizations that try to match on the dag