cmd/compile: in prove, shortcircuit self-facts

Sometimes, we can end up calling update with a self-relation
about a variable (x REL x). In this case, there is no need
to record anything: the relation is unsatisfiable if and only
if it doesn't contain eq.

This also helps avoiding infinite loop in next CL that will
introduce transitive closure of relations.

Passes toolstash -cmp.

Change-Id: Ic408452ec1c13653f22ada35466ec98bc14aaa8e
Reviewed-on: https://go-review.googlesource.com/100276
Reviewed-by: Austin Clements <austin@google.com>

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