i965/fs: Add support for sub-register byte offsets to the FS back-end IR.

i965/fs: Add support for sub-register byte offsets to the FS back-end IR.

It would be nice if we could have a single 'reg_offset' field
expressed in bytes that would serve the purpose of both, but the
semantics of 'reg_offset' are quite complex currently (it's measured
in units of one, eight or sixteen dwords depending on the register
file and the dispatch width) and changing it to bytes would be a very
intrusive change at this stage.  Add a separate 'subreg_offset' field
for now.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>