Fix DRR pattern when attributes and operands are interleaved and a dag subtree...

Fix DRR pattern when attributes and operands are interleaved and a dag subtree appears in the rewrite

This fixes an incorrect fatal error in TableGen. This code probably comes
from before attributes were allowed to interleave with operands in ODS.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D93915