Add a `-verify-roundtrip` option to `mlir-opt` intended to validate custom...

Add a `-verify-roundtrip` option to `mlir-opt` intended to validate custom printer/parser completeness

Running:

  MLIR_OPT_CHECK_IR_ROUNDTRIP=1 ninja check-mlir

will now exercises all of our test with a round-trip to bytecode and a comparison for equality.

Reviewed By: rriddle, ftynse, jpienaar

Differential Revision: https://reviews.llvm.org/D90088