[mlir] Add a shape function library op

Authored by jpienaar on Nov 28 2020, 3:53 PM.


[mlir] Add a shape function library op

Op with mapping from ops to corresponding shape functions for those op
in the library and mechanism to associate shape functions to functions.
The mapping of operand to shape function is kept separate from the shape
functions themselves as the operation is associated to the shape
function and not vice versa, and one could have a common library of
shape functions that can be used in different contexts.

Use fully qualified names and require a name for shape fn lib ops for
now and an explicit print/parse (based around the generated one & GPU
module op ones).

Differential Revision: https://reviews.llvm.org/D91672