[Triton-MLIR][Backend] Fix many problems to get the pipeline working (#809)

1. Rewrite code generation of insert_slice_async.
2. Correct the wrong index passed to extract_slice in pipeline.
3. Add a prologue in pipeline to wait for dangling cp.asyncs.  
4. Move scf to cf conversion inside TritonGPUToLLVM because we need to
perform membar before scf to cf. It shouldn't be a technical limitation
and could be improved by a more general membar analysis.
5. Use an attribute to memoize the shared memory size and support
dynamic shared memory.
6. Prevent the combine pass to reorder insert_slice and extract_slice
across async_wait

Co-authored-by: Superjomn <yanchunwei@outlook.com>

python/triton/compiler.py

@@ -875,7 +875,7 @@ def optimize_tritongpu_ir(mod, num_stages):
     pm.enable_debug()
     # Get error in backend due to wrong conversion in expanding async-related instruction.
     # TODO[Superjomn]: Open it when fixed.
-    # pm.add_tritongpu_pipeline_pass(num_stages)
+    pm.add_tritongpu_pipeline_pass(num_stages)
     pm.add_canonicalizer_pass()
     pm.add_cse_pass()
     pm.add_coalesce_pass()