Improve profile update in merge_blocks

Improve profile update in merge_blocks

When merging blocks we currently alway use count of the first basic block.
In some cases we merge block containing call to cold noreturn function (thus
having count 0 (reliable)) with earlier block with weaker form of profile.
In this case we can still preserve reliable count of 0.

The patch also makes block merging to pick higher of the counts if quality
is the same.  This should reduce chances of losing track of hot code in broken
profiles.

gcc/ChangeLog:

	* cfghooks.cc (merge_blocks): Choose more reliable or higher BB
	count.