[TableGen] Handle (outs variable_ops)

When `variable_ops` is specified in `InOperandList` of instruction,
it behaves as expected, i.e., does not count as operand.
So for `(ins variable_ops)` instruction description will have 0
operands.  However when used in OutOperandList it is counted as
operand. So `(outs variable_ops)` results in instruction with
one def.
This patch makes behavior of `variable_ops` in `out` list to match
that of `in` list.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D81095