Revert "[TableGen] Use cast instead of dyn_cast where result isn't checked...

Revert "[TableGen] Use cast instead of dyn_cast where result isn't checked before being dereferenced."

Turns out I misread the parentheses. Though I'm pretty sure its always a RecordRecTy and non of the callers really seem to expect null. But until I'm completely sure I'm going to revert this.

llvm-svn: 235469