i965: Use brw_reg's nr field to store register number.

i965: Use brw_reg's nr field to store register number.

In addition to combining another field, we get replace silliness like
"reg.reg" with something that actually makes sense, "reg.nr"; and no one
will ever wonder again why dst.reg isn't a dst_reg.

Moving the now 16-bit nr field to a 16-bit boundary decreases code size
by about 3k.

Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>