message: [spirv] Reorder `erase` and `emplace` to avoid "invalid iterator access".
  The iterator should be erased before adding a new entry
  into blockMergeInfo to avoid iterator invalidation.
  Closes
  COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/299 from denis0x0D:sandbox/reoder_erase
diff: @@ -1874,8 +1874,10 @@ LogicalResult ControlFlowStructurizer::structurizeImpl() {
  if (Block *mappedTo = mapper.lookupOrNull(newMerge))
  newMerge = mappedTo;
  - blockMergeInfo.try_emplace(newHeader, newMerge, newContinue);
  + // The iterator should be erased before adding a new entry into
  + // blockMergeInfo to avoid iterator invalidation.
  blockMergeInfo.erase(it);
  + blockMergeInfo.try_emplace(newHeader, newMerge, newContinue);
  }
  // The structured selection/loop's entry block does not have arguments.
  