MATCH: Add Max<Max<a,b>,a> -> Max<a,b> simplifcation

MATCH: Add Max<Max<a,b>,a> -> Max<a,b> simplifcation

This adds a simple match pattern to simplify
`max<max<a,b>,a>` to `max<a,b>`.  Reassociation handles
this already (r0-77700-ge969dbde29bfd396259357) but
seems like we should be able to handle this even before
reassociation.

This fixes part of PR tree-optimization/80574 but more
work is needed fix it the rest of the way. The original
testcase there is fixed but the RTL level is what fixes
it the rest of the way.

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

gcc/ChangeLog:

	* match.pd (minmax<minmax<a,b>,a>->minmax<a,b>): New
	transformation.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/reassoc-12.c: Disable all of
	the passes that enables match-and-simplify.
	* gcc.dg/tree-ssa/minmax-23.c: New test.