[PGO] Enhance pgo counter promotion

[PGO] Enhance pgo counter promotion

This is an incremental change to the promotion feature.

There are two problems with the current behavior:
1) loops with multiple exiting blocks are totally disabled
2) a counter update can only be promoted one level up in
  the loop nest -- which does help much for short trip
  count inner loops inside a high trip-count outer loops.

Due to this limitation, we still saw very large profile
count fluctuations from run to run for the affected loops
which are usually very hot.

This patch adds the support for promotion counters iteratively
across the loop nest. It also turns on the promotion for
loops with multiple exiting blocks (with a limit).

For single-threaded applications, the performance impact is flat
on average. For instance, dealII improves, but povray regresses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307863 91177308-0d34-0410-b5e6-96231b3b80d8