cmd/compile: in prove, fail fast when unsat is found

cmd/compile: in prove, fail fast when unsat is found

When an unsatisfiable relation is recorded in the facts table,
there is no need to compute further relations or updates
additional data structures.

Since we're about to transitively propagate relations, make
sure to fail as fast as possible to avoid doing useless work
in dead branches.

Passes toolstash -cmp.

Change-Id: I23eed376d62776824c33088163c7ac9620abce85
Reviewed-on: https://go-review.googlesource.com/100275
Reviewed-by: Austin Clements <austin@google.com>