i965/fs: Extend region width calculation to allow arbitrary execution sizes.

i965/fs: Extend region width calculation to allow arbitrary execution sizes.

Instead of just halving the execution size when the instruction is
compressed hoping that it will give a legal source region width, we
can calculate the maximum legal width value in closed form from the
component size and stride.  This makes sure that brw_reg_from_fs_reg()
always returns a valid hardware region even for virtual 32-wide
instructions (e.g. send-like instructions) that would seem to exceed
the hardware region width limit after halving.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>