[Patch] Teach genrecog/genoutput that scratch registers require write constraint...

[Patch] Teach genrecog/genoutput that scratch registers require write  constraint modifiers

gcc/

	* doc/md.texi (Modifiers): Consistently use "read/write"
	nomenclature rather than "input/output".
	* genrecog.c (constraints_supported_in_insn_p): New.
	(validate_pattern): If needed, also check constraints on
	MATCH_SCRATCH operands.
	* genoutput.c (validate_insn_alternatives): Catch earlyclobber
	operands with no '=' or '+' modifier.

From-SVN: r215388