Cap niter_for_unrolled_loop to upper bound

Cap niter_for_unrolled_loop to upper bound

For the reasons explained in PR77536, niter_for_unrolled_loop assumes 5
iterations in the absence of profiling information, although it doesn't
increase beyond the estimate for the original loop.  This left a hole in
which the new estimate could be less than the old one but still greater
than the limit imposed by CEIL (nb_iterations_upper_bound, unroll factor).

2017-05-04  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-ssa-loop-manip.c (niter_for_unrolled_loop): Add commentary
	to explain the use of truncating division.  Cap the number of
	iterations to the maximum given by nb_iterations_upper_bound,
	if defined.

gcc/testsuite/
	* gcc.dg/vect/vect-profile-1.c: New test.

From-SVN: r247591