TOSA-to-Linalg lowering for element-wise ops

- Wrote complete documentation for the `Broadcastable` op trait. This is mostly meant as a thorough description of its previous behavior, with the exception of minor feature updates.

- Restricted legality criteria for a `Broadcastable` op in order to simplify current and future lowering passes and increase efficiency of code generated by those passes. New restriction are: 1) A dynamic dimension in an inferred result is not compatible with a static dimension in the actual result. 2) Broadcast semantics are restricted to input operands and not supported between inferred and actual result shapes.

- Implemented TOSA-to-Linalg lowering support for unary, binary, tertiary element-wise ops. This support is complete for all legal cases described in the `Broadcastable` trait documentation.

- Added unit tests for `tosa.abs`, `tosa.add`, and `tosa.select` as examples of unary, binary, and tertiary ops.

Reviewed By: eric-k256

Differential Revision: https://reviews.llvm.org/D153291