Currently only A test operators had output, but since ABReducer and BCReducer
perform in-place replacement of A nodes into B nodes, and then sometimes into
C nodes, we need B and C nodes to have as many outputs. Otherwise, we run into
a situation where a node {x} has a use {y}, but its operators has 0 outputs,
which is inconsistent.

Bug: 
Change-Id: I0f73b83d2115dfeda3c9cbc97b9a2fc168f4c31b
Reviewed-on: https://chromium-review.googlesource.com/603716Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Alexandre Talon <alexandret@google.com>
Cr-Commit-Position: refs/heads/master@{#47202}