[MLIR] Fix tblgen properties printing to filter them out of discardable attrs dict (#79243)

This is to fix the bug reported in
https://discourse.llvm.org/t/whats-the-recommended-way-to-use-prop-dict/75921



When `prop-dict` is used in the assembly format, `attr-dict` should
print discardable attributes only.

Co-authored-by: Arthurq Qiu <arthurq@nvidia.com>