[mlir] DialectConversion: fix OperationLegalizer::isIllegal result when...

[mlir] DialectConversion: fix OperationLegalizer::isIllegal result when legality callback returns None

OperationLegalizer::isIllegal returns false if operation legality wasn't
registered by user and we expect same behaviour when dynamic legality
callback return None, but instead true was returned.

Differential Revision: https://reviews.llvm.org/D113267