solver: do_map remerbers now precedent Benchmark used

Switch do_bench from a single-benchmark view (i.e., do_bench is the error rate for instr1, instr2, bench) to multiple benchmarks (e.g. instr1, instr2, bench1, ...). Also correcting a bug/feature when do_bench value was reuse as soon as an error rate for {instr1, instr2} was already computed.