[XLA:GPU] Add a flag to create new Triton fusions in Priority Fusion pass.
This also changes a bit how Priority Fusion works with Triton fusion:
* `--xla_gpu_experimental_enable_triton_softmax_priority_fusion` enables `SoftmaxRewriterTriton`, but doesn't affect `PriorityFusion`.
* By default Priority Fusion doesn't create new Triton fusions. If there is a Generic Triton fusion that was created before, Priority Fusion will try to fuse more into it regardless of flags. Current default state is that there are no generic Triton fusions created in the pipeline (only GEMMs), so it doesn't affect end users.
* With `--xla_gpu_experimental_enable_triton_heroless_priority_fusion`, `PriorityFusion` will try to tile any new producer-consumer fusion and target to Triton.
PiperOrigin-RevId: 689335386