i965/nir: Use offset() instead of altering reg_offset directly.

i965/nir: Use offset() instead of altering reg_offset directly.

offset() properly handles reg_width, so it'll work for SIMD16.

While we're in the area, simplify a few cases, and use retype() to cut a
few more lines of code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>