[Triton-MLIR][BACKEND] Fix the membar pass to add missing barriers caused by scf.for (#933)

1. Add missing barriers and revert the previous temporary solution
2. Extract the `run` method from membar analysis because the membar
analysis should have two phases, including construction, which doesn't
modify any IR, and modification, which adds barrier IRs. Hope this could
make the use of membar clear.

lib/Dialect/TritonGPU/Transforms/Prefetch.cpp

@@ -130,10 +130,10 @@ LogicalResult Prefetcher::initialize() {
 
   if (dotsInFor.empty())
     return failure();
-  
+
   // TODO: segfault (original for still has uses)
   // when used in flash attention that has 2 dots in the loop
-  if(dotsInFor.size() > 1)
+  if (dotsInFor.size() > 1)
     return failure();
 
   // returns source of cvt