R600/SI: Remove vaddr operand from BUFFER_LOAD_*_OFFSET instructions

This operand is never used.
R600/SI: Specify wavefront size for SI and CI
R600/SI: Use scratch memory for large private arrays
R600/SI: Clean up some of the unused REGISTER_{LOAD,STORE} code

There are a few more cleanups to do, but I ran into some problems
with ext loads and trunc stores, when I tried to change some of the
vector loads and stores from custom to legal, so I wasn't able to
get rid of everything.