Replace conditional_replacement with match and simplify

Replace conditional_replacement with match and simplify

This is the first of series of patches to simplify phi-opt
to use match and simplify in many cases.  This simplification
will more things to optimize.

This is what Richard requested in
https://gcc.gnu.org/pipermail/gcc-patches/2021-May/571197.html
and I think it is the right thing to do too.

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

gcc/ChangeLog:

	PR tree-optimization/25290
	* tree-ssa-phiopt.c (match_simplify_replacement):
	New function.
	(tree_ssa_phiopt_worker): Use match_simplify_replacement.
	(two_value_replacement): Change the comment about
	conditional_replacement.
	(conditional_replacement): Delete.