Make inliner loop hints more agressive

Make inliner loop hints more agressive

This patch makes loop inline hints more agressive.  If we know iteration
count or stride, we currently assume improvement in time relative to
preheader count.  I changed it to header count, since this knowledge
is supposed to likely help unrolling and vectorizing which brings
benefits relative to that.

	* ipa-fnsummary.cc (analyze_function_body): For loop
	heuristics use header count instead of preheader count.