amdgcn: Add [us]mulsi3_highpart SGPR alternatives

amdgcn: Add [us]mulsi3_highpart SGPR alternatives

This patch splits the <su>mulsi3_highpart pattern into an expander and
register/immediate alternatives (to avoid meaningless sign/zero_extends on
constants), and adds alternatives for SGPR high-part multiply instructions
on GCN5+.

2021-06-29  Julian Brown  <julian@codesourcery.com>

gcc/
	* config/gcn/gcn.md (<su>mulsi3_highpart): Change to expander.
	(<su>mulsi3_highpart_reg, <su>mulsi3_highpart_imm): New patterns.