nir: Add an interface for the builder to insert instructions before.

nir: Add an interface for the builder to insert instructions before.

So far we'd only used nir_builder to build brand new programs.  But if
we're doing modifications to instructions (like in a lowering pass), then
we want to generate new stuff before the instruction we're modifying.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>