[XLA:GPU] Add an optional compiler pass that verifies Triton fusions.
[XLA:GPU] Add an optional compiler pass that verifies Triton fusions.

The pass uses existing autotuning logic to compile and run two versions of all `FusionInstructions` with a Triton backend:
- The original instruction with the Triton Backend
- A modified instruction without a backend. This one will be compiled by the regular emitters.
The outs of the two versions are compared and the pass fails if there is a mismatch.
