Relax VIEW_CONVERT_EXPR - CONVERT_EXPR combination

2017-05-25  Marc Glisse  <marc.glisse@inria.fr>

gcc/
	* match.pd (view_convert (convert@0 @1)): Handle zero-extension.

gcc/testsuite/
	* gcc.dg/tree-ssa/vce-1.c: New file.

From-SVN: r248449