PHIOPT: Fix minmax_replacement for three way

PHIOPT: Fix minmax_replacement for three way

So when diamond bb support was added to minmax_replacement in r13-1950-g9bb19e143cfe,
the code was not expecting the alt_middle_bb not to exist if it was empty (for threeway_p).
So when factor_out_conditional_conversion was used to factor out conversions, it turns out
the assumption for alt_middle_bb to be wrong and we ended up with threeway_p being true but
having middle_bb being empty but alt_middle_bb not being empty which causes wrong code in
many cases.

This patch fixes the issue by adding a test for the 2 cases where the assumption on
threeway_p case having the other bb being empty.

Changes made:
v2: Fix test for `(a <= u) b = MAX(a, d) else b = u`.

Note my plan for GCC 15 is remove minmax_replacement as match.pd will catch all cases
at that point.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

	PR tree-optimization/111469

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (minmax_replacement): Fix
	the assumption for the `non-diamond` handling cases
	of diamond code.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/execute/pr111469-1.c: New test.