nv50/ir: treat addresses as local

nv50/ir: treat addresses as local

Address registers are always loaded right before use. Don't treat them
as "global", which will cause them to be put into the function's
linkage, and will make the register allocator hold onto that
register until the end of the function.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>