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

name                               old time/op  new time/op  delta
BM_HloDfsReachabilityBuild/1        105ns ± 1%   105ns ± 1%     ~     (p=0.151 n=5+5)
BM_HloDfsReachabilityBuild/64      3.51µs ± 3%  2.38µs ± 2%  -32.18%  (p=0.008 n=5+5)
BM_HloDfsReachabilityBuild/128     6.93µs ± 2%  4.79µs ± 3%  -30.82%  (p=0.008 n=5+5)
BM_HloDfsReachabilityBuild/256     13.8µs ± 1%   9.5µs ± 1%  -30.68%  (p=0.008 n=5+5)
BM_HloDfsReachabilityBuild/512     28.0µs ± 1%  19.3µs ± 1%  -30.86%  (p=0.008 n=5+5)
BM_HloDfsReachabilityBuild/4096     268µs ± 1%   188µs ± 1%  -29.90%  (p=0.008 n=5+5)
BM_HloDfsReachabilityBuild/32768   2.74ms ± 3%  2.03ms ± 2%  -25.99%  (p=0.008 n=5+5)
BM_HloDfsReachabilityBuild/262144  43.8ms ± 2%  38.9ms ± 2%  -11.15%  (p=0.008 n=5+5)
