i965/vec4: Drop backend_reg::in_range() in favor of regions_overlap().

i965/vec4: Drop backend_reg::in_range() in favor of regions_overlap().

This makes sure that overlap checks are done correctly throughout the
back-end when the '*this' register starts before the register/size
pair provided as argument, and is actually less annoying to use than
in_range() at this point since regions_overlap() takes its size
arguments in bytes.

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