Fix ICE in lsplit when built with -O3 -fno-guess-branch-probability [PR103793]

Fix ICE in lsplit when built with -O3 -fno-guess-branch-probability [PR103793]

no-guess-branch-probability option requires profile_count scaling with
initialized_p guard, use multiply instead of apply_scale, which will do
the right thing to undefined probabilities and will not cause unnecesary
roundoff errors and precision info loss.
Also merge the missed part of r12-6086 of factor out function to avoid
duplicate code.

Regression testest pass on Power and X86.

gcc/ChangeLog:

	PR tree-optimization/103793
	* tree-ssa-loop-split.c (fix_loop_bb_probability): New function.
	(split_loop): Use multiply to scale loop1's exit probability.
	(do_split_loop_on_cond): Call fix_loop_bb_probability.

gcc/testsuite/ChangeLog:

	PR tree-optimization/103793
	* gcc.dg/pr103793.c: New test.