GCN machine description

GCN machine description

This patch contains the machine description portion of the GCN back-end.  I've
broken it out mainly to avoid the mailing list size limit.

2019-01-17  Andrew Stubbs  <ams@codesourcery.com>
	    Kwok Cheung Yeung  <kcy@codesourcery.com>
	    Julian Brown  <julian@codesourcery.com>
	    Tom de Vries  <tom@codesourcery.com>
	    Jan Hubicka  <hubicka@ucw.cz>
	    Martin Jambor  <mjambor@suse.cz>

	gcc/
	* config/gcn/constraints.md: New file.
	* config/gcn/gcn-valu.md: New file.
	* config/gcn/gcn.md: New file.
	* config/gcn/predicates.md: New file.

Co-Authored-By: Jan Hubicka <hubicka@ucw.cz>
Co-Authored-By: Julian Brown <julian@codesourcery.com>
Co-Authored-By: Kwok Cheung Yeung <kcy@codesourcery.com>
Co-Authored-By: Martin Jambor <mjambor@suse.cz>
Co-Authored-By: Tom de Vries <tom@codesourcery.com>
From-SVN: r268022