i965: Use implicit backend_reg copy-constructor.

i965: Use implicit backend_reg copy-constructor.

In order to do this, we have to change the signature of the
backend_reg(brw_reg) constructor to take a reference to a brw_reg in
order to avoid unresolvable ambiguity about which constructor is
actually being called in the other modifications in this patch.

As far as I understand it, the rule in C++ is that if multiple
constructors are available for parent classes, the one closest to you in
the class heirarchy is closen, but if one of them didn't take a
reference, that screws things up.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>