[polly] scop detection: detect regions bottom up

[Sebastian Pop]

Tobias Grosser wrote:
> I am still missing the overall compile-time to understand many
> percent of the compile-time is actually spent in Polly. If -O3 takes
> 10 seconds, scop detection is super slow. if -O3 takes 10^1000
> seconds, scop detection is actually amazingly fast, but we just
> don't find anything to optimize.

I already have posted the longest running scop detection line before:

  ---User Time---     --System Time--  --User+System--    ---Wall Time---    --- Name ---
  289.6700 ( 69.6%)   1.6400 ( 41.8%)  291.3100 ( 69.4%)  292.6551 ( 69.2%)  Polly - Detect static control parts (SCoPs)

With all the changes I posted, there are fewer lines like these, however I can
still see several files in which scop detection takes more than 30% of the
overall compile time.  Those files seem to be heavily templated and using STL.

Sebastian
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