MATCH: Move `a <= CST1 ? MAX<a, CST2> : a` optimization to match

MATCH: Move `a <= CST1 ? MAX<a, CST2> : a` optimization to match

This moves the `a <= CST1 ? MAX<a, CST2> : a` optimization
from phiopt to match. It just adds a new pattern to match.pd.

There is one more change needed before being able to remove
minmax_replacement from phiopt.

A few notes on the testsuite changes:
* phi-opt-5.c is now able to optimize at phiopt1 so remove
the xfail.
* pr66726-4.c can be optimized during fold before phiopt1
so need to change the scanning.
* pr66726-5.c needs two phiopt passes currently to optimize
to the right thing, it needed 2 phiopt passes before, the cast
from int to unsigned char is the reason.
* pr66726-6.c is what the original pr66726-4.c was testing
before the fold was able to optimize it.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* match.pd (`(a CMP CST1) ? max<a,CST2> : a`): New
	pattern.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/phi-opt-5.c: Remove last xfail.
	* gcc.dg/tree-ssa/pr66726-4.c: Change how scanning
	works.
	* gcc.dg/tree-ssa/pr66726-5.c: New test.
	* gcc.dg/tree-ssa/pr66726-6.c: New test.