nir: Add a lower_fdiv option, turn fdiv into fmul/frcp.

nir: Add a lower_fdiv option, turn fdiv into fmul/frcp.

The nir_opt_algebraic rule

(('fadd', ('flog2', a), ('fneg', ('flog2', b))), ('flog2', ('fdiv', a, b))),

can produce new fdiv operations, which need to be lowered on i965,
as we don't actually implement fdiv.  (Normally, we handle this in
GLSL IR's lower_instructions pass, but in the above case we introduce
an fdiv after that point.  So, make NIR do it for us.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: mesa-stable@lists.freedesktop.org