ir_to_mesa: Remove the "ir_to_mesa_" prefix on src_reg/dst_reg types.

ir_to_mesa: Remove the "ir_to_mesa_" prefix on src_reg/dst_reg types.

Both classes are completely private to ir_to_mesa.cpp, so there won't be
any name conflicts with other parts of Mesa.  The prefix simply makes it
harder to read.

Also, use a class rather than typedef structs.

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