perf diff: Use internal rb tree for hists__precompute

perf diff: Use internal rb tree for hists__precompute

There's missing change for hists__precompute to iterate either
entries_collapsed or entries_in tree. The change was initiated
for hists_compute_resort function in commit:

  66f97ed perf diff: Use internal rb tree for compute resort

but was missing for hists__precompute function changes.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1355404152-16523-2-git-send-email-jolsa@redhat.com
[ committer note: Reduce patch size, no functional change ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>