Add check_effective_target_autoincdec.

Add check_effective_target_autoincdec.

Add check_effective_target_autoincdec that returns true if a target
runs the auto_inc_dec optimization pass.

    gcc/
	* doc/sourcebuild.texi (autoincdec): Add autoincdec description.

    gcc/testsuite/
	PR middle-end/46932
	* gcc.dg/pr46932.c: Use dg-require-effective-target autoincdec.
	* lib/target-supports.exp: Add check_effective_target_autoincdec.

From-SVN: r251087