i965/vec4: Convert src_reg/dst_reg to brw_reg at the end of the visitor.

i965/vec4: Convert src_reg/dst_reg to brw_reg at the end of the visitor.

This patch makes the visitor convert registers to the HW_REG file at the
very end, after register allocation, post-RA scheduling, and dependency
control flagging.  After that, everything is in fixed brw_regs.

This simplifies the code generator, as it can just use the hardware
registers rather than having to interpret our abstract files.  In
particular, interpreting the UNIFORM file meant reading prog_data
to figure out where push constants are supposed to start.

Having the part of the code that performs register allocation also
translate everything to hardware registers seems sensible.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>