i965/vec4: Replace vec4_instruction::regs_read with ::size_read using byte units.

i965/vec4: Replace vec4_instruction::regs_read with ::size_read using byte units.

The previous regs_read value can be recovered by rewriting each
reference of regs_read() like 'x = i.regs_read(j)' to 'x =
DIV_ROUND_UP(i.size_read(j), reg_unit)'.

For the same reason as in the previous patches, this doesn't attempt
to be particularly clever about simplifying the result in the interest
of keeping the rather lengthy patch as obvious as possible.  I'll come
back later to clean up any ugliness introduced here.

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