[tablegen] Fixed few !foreach evaluation issues.

* !foreach on lists didn't evaluate operands of the RHS operator.
  This made nested operators silently fail.
* A typo in the code could result in a wrong value substituted
  for an operation which produced a false '!foreach requires an operator' error.
* Keep recursion over the DAG within ForeachHelper. This simplifies
  things a bit as we no longer need to pass the Type around in order
  to prevent recursion.

Differential Revision: https://reviews.llvm.org/D43083

llvm-svn: 324758