[mlir][spirv] Use MapVector for BlockMergeInfoMap (llvm#169636)

This should ensure that the structurizer while loop is deterministic
across runs. Use of `MapVector` addresses the source of the
nondeterminism which is use of a `Block*` as a map key.

fixes llvm#128547

Author: IgWod-IMG

mlir/lib/Target/SPIRV/Deserialization/Deserializer.cpp

@@ -2923,9 +2923,6 @@ LogicalResult spirv::Deserializer::structurizeControlFlow() {
     return failure();
   }
 
-  // TODO: This loop is non-deterministic. Iteration order may vary between runs
-  // for the same shader as the key to the map is a pointer. See:
-  // https://github.com/llvm/llvm-project/issues/128547
   while (!blockMergeInfo.empty()) {
     Block *headerBlock = blockMergeInfo.begin()->first;
     BlockMergeInfo mergeInfo = blockMergeInfo.begin()->second;

mlir/lib/Target/SPIRV/Deserialization/Deserializer.h

@@ -58,7 +58,9 @@ struct DebugLine {
 };
 
 /// Map from a selection/loop's header block to its merge (and continue) target.
-using BlockMergeInfoMap = DenseMap<Block *, BlockMergeInfo>;
+/// Use `MapVector<>` to ensure a deterministic iteration order with a pointer
+/// key.
+using BlockMergeInfoMap = llvm::MapVector<Block *, BlockMergeInfo>;
 
 /// A "deferred struct type" is a struct type with one or more member types not
 /// known when the Deserializer first encounters the struct. This happens, for