tree-optimization/107919 - uninit diagnostic predicate simplification

We fail to simplify

        ((_145 != 0B) AND (_531 == 2) AND (_109 == 0))
        OR ((NOT (_145 != 0B)) AND (_531 == 2) AND (_109 == 0))
        OR ((NOT (_531 == 2)) AND (_109 == 0))

because the existing simplification of !A && B || A && B is implemented
too simplistic.  The following re-implements that which fixes the
bogus uninit diagnostic when using -O1 but not yet at -O2.

	PR tree-optimization/107919
	* gimple-predicate-analysis.cc (predicate::simplify_2):
	Handle predicates of arbitrary length.

	* g++.dg/warn/Wuninitialized-pr107919-1.C: New testcase.