[mlir][Linalg] Enable fusion of std.constant (producer) with

linalg.indexed_generic (consumer) with tensor arguments.

The implementation of fusing std.constant producer with a
linalg.indexed_generic consumer was already in place. It is exposed
with this change. Also cleaning up some of the patterns that implement
the fusion to not be templated, thereby avoiding lot of conditional
checks for calling the right instantiation.

Differential Revision: https://reviews.llvm.org/D84566