i965: Make sure that backend_reg::type and brw_reg::type are consistent for fixed...

i965: Make sure that backend_reg::type and brw_reg::type are consistent for fixed regs.

And define non-mutating helper functions to retype fixed and normal
regs with a common interface.  At some point we may want to get rid of
::fixed_hw_reg completely and have fixed regs use the normal register
data members (e.g. backend_reg::reg to select a fixed GRF number,
src_reg::swizzle to store the swizzle, etc.), I have the feeling that
this is not the last headache we're going to get because of the
multiple ways to represent the same thing and the different register
interface depending on the file a register is stored in...

Reviewed-by: Paul Berry <stereotype441@gmail.com>