When I forward ported the scatter/gather patterns from my GCC 9 
implementation I didn't notice that GCC 10 has a different naming 
scheme. :-(

The patterns were being used because all GCN vector loads end up being 
scatter/gather, but not by the actual vectorizer. The test fails were 
there to see, but there are still a lot of those to work through.

This patch uses the new two-mode naming scheme and implements the 
offsets correctly. This is actually a step forward for GCN because the 
offsets are always SImode, regardless of the primary mode.

Andrew