Disable loop distribution for loops with estimated iterations 0

Disable loop distribution for loops with estimated iterations 0

This prevents useless loop distribiton produced in hmmer.  With FDO we now
correctly work out that the loop created for last iteraiton is not going to
iterate however loop distribution still produces a verioned loop that has no
chance to survive loop vectorizer since we only keep distributed loops
when loop vectorization suceeds and it requires number of (header) iterations
to exceed the vectorization factor.

gcc/ChangeLog:

	* tree-loop-distribution.cc (loop_distribution::execute): Disable
	distribution for loops with estimated iterations 0.