i965/vec4: Port regions_overlap() to the vec4 IR.

i965/vec4: Port regions_overlap() to the vec4 IR.

This is copy-pasted almost line by line from the FS back-end.  The
only reason it cannot be implemented in terms of backend_reg is that
the backend_reg::nr field doesn't have the same meaning for uniforms
on both back-ends.  It could be easily deduplicated by using a
template function.

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