i965/fs: Simplify and fix register offset calculation of try_copy_propagate().

i965/fs: Simplify and fix register offset calculation of try_copy_propagate().

try_copy_propagate() was special-casing UNIFORM registers (the
BAD_FILE, ARF and FIXED_GRF cases are dead, see the assertion at the
top of the function) and then failing to take into account the
possibility of the instruction reading from a non-zero offset of the
destination of the copy.  The VGRF/ATTR handling takes it into account
correctly, and there is no reason we couldn't use the exact same logic
for the UNIFORM file aside from the fact that uniforms represent
reg_offset in different units.  We can work around that easily by
defining an additional constant with the right unit reg_offset is
expressed in.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>