TableGen: Use DefInit::getDef() instead of the type's getRecord()

The former simply makes more sense: we want to access the data here in
the backend, not information about the type.

More importantly, removing users of RecordRecTy::getRecord() allows us
more freedom to refactor the frontend.

Change-Id: Iee8905fd22cdb9b11c42ca03246c03d8fe4dd77f
llvm-svn: 326699