Fix tree-ssa/update-cunroll.c

Fix tree-ssa/update-cunroll.c

In this testcase the profile is misupdated before loop has two exits.
The first exit is one eliminated by complete unrolling while second exit remains.
We remove first exit but forget about fact that the source BB of other exit will
then have higher frequency making other exit more likely.

This patch fixes that in duplicate_loop_body_to_header_edge.
While looking into resulting profiles I also noticed that in some cases
scale_loop_profile may drop probabilities to 0 incorrectly either when
trying to update exit from nested loop (which has similar problem) or when the profile
was inconsistent as described in coment bellow.

gcc/ChangeLog:

	PR middle-end/110590
	* cfgloopmanip.cc (scale_loop_profile): Avoid scaling exits within
	inner loops and be more careful about inconsistent profiles.
	(duplicate_loop_body_to_header_edge): Fix profile update when eliminated
	exit is followed by other exit.

gcc/testsuite/ChangeLog:

	PR middle-end/110590
	* gcc.dg/tree-prof/update-cunroll-2.c: Remove xfail.
	* gcc.dg/tree-ssa/update-cunroll.c: Likewise.