ir_to_mesa: Use constructors to convert between src_reg and dst_reg.

ir_to_mesa: Use constructors to convert between src_reg and dst_reg.

Rather than ir_to_mesa_dst_reg_from_src and ir_to_mesa_src_reg_from_dst.

The new constructors are marked 'explicit' so that the compiler can
catch cases where source and destination registers were accidentally
interchanged.

This also necessitated using constructors to initialize the undef and
address registers, as well as adding a default constructor.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>