hlo_reachability: reserve indices' storage at Build() time
hlo_reachability: reserve indices' storage at Build() time

name                             old time/op  new time/op   delta
BM_HloReachabilityBuild/1         165ns ± 0%    167ns ± 0%   +0.88%  (p=0.016 n=5+4)
BM_HloReachabilityBuild/64       6.27µs ± 1%   5.25µs ± 1%  -16.18%  (p=0.008 n=5+5)
BM_HloReachabilityBuild/128      12.9µs ± 3%   11.0µs ± 4%  -15.11%  (p=0.008 n=5+5)
BM_HloReachabilityBuild/256      27.3µs ± 2%   23.4µs ± 2%  -14.06%  (p=0.008 n=5+5)
BM_HloReachabilityBuild/512      60.0µs ± 2%   51.9µs ± 1%  -13.55%  (p=0.008 n=5+5)
BM_HloReachabilityBuild/4096      974µs ± 1%    901µs ± 1%   -7.45%  (p=0.008 n=5+5)
BM_HloReachabilityBuild/32768    42.5ms ± 1%   41.1ms ± 2%   -3.17%  (p=0.008 n=5+5)
BM_HloReachabilityBuild/262144    5.19s ±50%  11.10s ±159%     ~     (p=0.548 n=5+5)
