[PATCH] Introduce new param: AVG_LOOP_NITER

[Martin Liška]

Hello.

During investigation of an IVOPTs issue, I noticed that tree-ssa-loop-ivopts.c uses a hard-coded
constant AVG_LOOP_NITER. Very similar constant can be seen in cfgloopanal.c. Thus, I've changed
that to a new param.

Apart from that, I would like to change the default value to 10.
As numbers of SPEC CPU2006 show:

Loop count: 18236
  avg. # of iter: 11908.46
  median # of iter: 10.00
  avg. (1% cutoff) # of iter: 1003.87
  avg. (5% cutoff) # of iter: 449.78
  avg. (10% cutoff) # of iter: 209.46
  avg. (20% cutoff) # of iter: 62.24
  avg. (30% cutoff) # of iter: 42.69

Even though the average number of loop iteration is quite high, let's start to increase it
conservatively to 10. Benchmark results are within a noise level (-Ofast runs faster by 0.47%).

Patch survives regression tests and bootstraps on x86_64-linux-gnu.
Ready for trunk?
Thanks,
Martin
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