hlo_reachability_test: add benchmarks for HloReachabilityMap
hlo_reachability_test: add benchmarks for HloReachabilityMap

Add two microbenchmarks: one for BitSet's operator|= (a.k.a. Union)
and another for its main user, HloReachabilityMap::Build.

Having these two will allow us to evaluate changes to operator|='s
implementation.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/tensorflow/pull/62647 from Intel-tensorflow:othakkar/pattern_matcher_control_inouts 4a7cea22083f22a4e4fb26e05707b5961ae3c67f
