[mlir][Linalg] Define a linalg.init_tensor operation.

This operation is used to materialize a tensor of a particular
shape. The shape could be specified as a mix of static and dynamic
values.

The use of this operation is to be an `init` tensor for Linalg
structured operation on tensors where the bounds of the computation
depends on the shape of the output of the linalg operation. The result
of this operation will be used as the `init` tensor of such Linalg
operations. To note,

1) The values in the tensor materialized is not used. Any operation to
   which this is an init tensor is expected to overwrite the entire
   tensor.
2) The tensor is materialized only for the shape of the output and to
   make the loop bounds depend only on operands of the structured
   operation.

Based on (1) and (2) it is assumed that these operations eventually go
away since they are only used in `dim` operations that can be
canonicalized to make this operation dead. Such canonicalization are
added here too.

Differential Revision: https://reviews.llvm.org/D93374