Before this CL, inequality was recorded in a bit matrix using
SSA IDs. This allowed to record inequality for SSA values that
we didn't know any relation in the partial order of. Unfortunately,
this also means that inequality is harder to use within the poset
itself as there is not fast way to map from internal poset indices
and SSA values.

Since we will need to check for inequality in following CLs within
code that lost track of SSA values, switch to use a bit matrix
of poset indices instead. This requires always allocate a poset
node (as a new root) for values that are first seen in a SetNonEqual
call, but it doesn't sound like a big problem. The other solution
(creating and maintaining a reverse map from poset indices to SSA
values) seem more complicated and memory hungry.

Change-Id: Ic917485abbe70aef7ad6fa98408e5430328b6cd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/196782
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>