i965/fs: Add wrapper functions for fs_inst::regs_read and ::regs_written.

i965/fs: Add wrapper functions for fs_inst::regs_read and ::regs_written.

This is in preparation for dropping fs_inst::regs_read and
::regs_written in favor of more accurate alternatives expressed in
byte units.  The main reason these wrappers are useful is that a
number of optimization passes implement dataflow analysis with
register granularity, so these helpers will come in handy once we've
switched register offsets and sizes to the byte representation.  The
wrapper functions will also make sure that GRF misalignment (currently
neglected by most of the back-end) is taken into account correctly in
the calculation of regs_read and regs_written.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>