[polly] scop detection: detect regions bottom up

[Sebastian Pop]

Star Tan wrote:
> the top 15 expensive compile passes and the ScopDetect pass only accounts for
> 2.1% for compiling doitgen.ll (Table 3) and 6.9% for compiling jmemmgr.ll
> (Table 4).  However, Sebastian's testing shows that his patch file to the
> ScopDetect pass can reduce the compile time by 25%! 

With all the scop detection patches I have posted, we are down by about 10x in
compile time used by Polly, and I think there still are about 1000 seconds to
cut down from scop detection compile time.

Sebastian
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