[TableGen] Prevent invalid code generation when emitting AssemblerPredicate conditions.

Summary:
The loop which emits AssemblerPredicate conditions also links them together by emitting a '&&'.
If the 1st predicate is not an AssemblerPredicate, while the 2nd one is, nothing gets emitted for the 1st one, but we still emit the '&&' because of the 2nd predicate.
This generated code looks like "( && Cond2)" and is invalid.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D8294

llvm-svn: 234312