[backend] Ensure a deterministic allocation order of scratch smem buffers (triton-lang#5595)

I was seeing non-deterministic SMEM allocation sizes and offsets across
compilations with same source and compiler. It seems due to scratch
buffers processed in a non-deterministic order, due to the use of
`DenseMap`. Using `MapVector` should fix it.

Author: htyu

include/triton/Analysis/Allocation.h

@@ -209,7 +209,7 @@ class Allocation {
   };
 
   /// Op -> Scratch Buffer
-  using OpScratchMapT = DenseMap<Operation *, BufferT *>;
+  using OpScratchMapT = llvm::MapVector<Operation *, BufferT *>;
   /// Value -> Explicit Buffer
   using ValueBufferMapT = llvm::MapVector<Value, BufferT *>;
   /// Value -> Alias Buffer