amdgcn: Fix wrong-code bug in 64-bit masked add

2020-04-24  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/gcn/gcn-valu.md (add<mode>_zext_dup2_exec): Fix merge
	of high-part.
	(add<mode>_sext_dup2_exec): Likewise.