nir: Add a couple quick-and-dirty out-of-SSA helpers

nir: Add a couple quick-and-dirty out-of-SSA helpers

These are designed for use within an optimization pass when SSA becomes
more pain than it's worth.  They're very naive and don't generate
anything close to optimal register-based NIR.  Also, they may result in
shaders which do not validate because of, for instance, registers in phi
sources.  However, the register-based into-SSA pass should be pretty
efficient at cleaning up the mess.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>