amdgcn: Use correct offset mode for gather/scatter

amdgcn: Use correct offset mode for gather/scatter

The scatter/gather pattern names changed for GCC 10, but I hadn't noticed.
This switches the patterns to the new offset mode scheme.

2020-02-21  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/gcn/gcn-valu.md (gather_load<mode>): Rename to ...
	(gather_load<mode>v64si): ... this and set operand 2 to V64SI.
	(scatter_store<mode>): Rename to ...
	(scatter_store<mode>v64si): ... this and set operand 1 to V64SI.
	(scatter<mode>_exec): Delete. Move contents ...
	(mask_scatter_store<mode>): ... here, and rename that to ...
	(mask_gather_load<mode>v64si): ... this. Set operand 2 to V64SI.
	Remove mode conversion.
	(mask_gather_load<mode>): Rename to ...
	(mask_scatter_store<mode>v64si): ... this. Set operand 1 to V64SI.
	Remove mode conversion.
	* config/gcn/gcn.c (gcn_expand_scaled_offsets): Remove mode conversion.