value-prof.cc: Correct edge prob calculation.

The mod-subtract optimization with ncounts==1 produced incorrect edge
probabilities due to incorrect conditional probability calculation. This
patch fixes the calculation.

Signed-off-by: Filip Kastl <filip.kastl@gmail.com>

gcc/ChangeLog:

	* value-prof.cc (gimple_mod_subtract_transform): Correct edge
	prob calculation.