[TableGen] Make sure !if is evaluated throughout class inheritance.

Without the patch !if() is only evaluated if it's used directly.
If it's passed through more than one level of class inheritance,
we end up with a reference to an anonymous record with unresolved
references to the original arguments !if may have used.

The root cause of the problem is that TernOpInit::isComplete()
was always returning false and that prevented use of the folded
value of !if() as an initializer for the record at the next level
of inheritance.

Differential Revision: https://reviews.llvm.org/D42695

llvm-svn: 323807