GCN vector sizes range between 64 and 512 bytes, none of which have
correspondingly sized integer modes.  This breaks a number of assumptions
throughout the compiler, but I don't really want to create modes just for this
purpose.

Instead, this patch fixes up the cases that I've found, so far, such that the
compiler tries something else, or fails to optimize, rather than just ICE.

2018-11-15  Andrew Stubbs  <ams@codesourcery.com>
            Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* tree-vect-stmts.c (vectorizable_store): Don't ICE when
	int_mode_for_size fails.
	(vectorizable_load): Likewise.

Co-Authored-By: Kwok Cheung Yeung <kcy@codesourcery.com>

From-SVN: r266190