nir/flrp: Add new lowering pass for flrp instructions

nir/flrp: Add new lowering pass for flrp instructions

This pass will soon grow to include some optimizations that are
difficult or impossible to implement correctly within nir_opt_algebraic.
It also include the ability to generate strictly correct code which the
current nir_opt_algebraic lowering lacks (though that could be changed).

v2: Document the parameters to nir_lower_flrp.  Rebase on top of
3766334923e ("compiler/nir: add lowering for 16-bit flrp")

Reviewed-by: Matt Turner <mattst88@gmail.com>