tree-optimization/109154 - improve if-conversion for vectorization

tree-optimization/109154 - improve if-conversion for vectorization

With multi-argument PHIs and now doing VN on the if-converted blocks
the optimization of CSEing condition and negated condition doesn't
work well anymore.  The following restores this a little bit for
the case of a single inverted condition into a COND_EXPR where
we can instead swap the COND_EXPR arms.  The same optimization
is already done for the case of two-argument PHIs.

This avoids one comparison/mask for the testcase at hand.

	PR tree-optimization/109154
	* tree-if-conv.cc (gen_phi_arg_condition): Handle single
	inverted condition specially by inverting at the caller.
	(gen_phi_arg_condition): Swap COND_EXPR arms if requested.