tree-optimization/106860 - fix profile scaling in split_loop

tree-optimization/106860 - fix profile scaling in split_loop

The following fixes a mistake in loop splitting which assumes loop
latches have a single predecessor and that edge is from the exit
test.  Instead work from the single exit edge we have to find the
edge towards the latch.

	PR tree-optimization/106860
	* tree-ssa-loop-split.cc (split_loop): Find the exit to
	latch edge from the loop exit edge instead of from the
	latch.  Verify we're going to find it.

	* g++.dg/opt/pr106860.C: New testcase.

(cherry picked from commit 0386609923577e07354ee63754795b2f729e7e00)