nir/builder: Add a vector extract helper

This one's a tiny bit better than what we had in spirv_to_nir because it
emits a binary tree rather than a linear walk.  It also doesn't leave
around unneeded bcsel instructions for a constant index and returns an
undef for constant OOB access.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit fe9a6c0f)