Add maximum NSA size limit as an ISA feature.
Use this to reduce NSA usage on GFX10.1 to avoid stability issues
with 4 and 5 dwords NSA instructions.
Maintain use of longer NSA instructions on GFX10.3.
Note: this also contains some minor fixes for GlobalISel which
did not work correctly with non-NSA form instructions on GFX10.
It seems a bit odd to define it like this gives the impression that the limit varies on different subtargets by design, which it doesn't really. The only reason to limit it to 5 is to "avoid stability issues" (i.e. work around hardware bugs). But I'm not sure what else to suggest.