nir: remove fnot/fxor/fand/for opcodes

There doesn't seem to be any reason to keep these opcodes around:
* fnot/fxor are not used at all.
* fand/for are only used in lower_alu_to_scalar, but easily replaced

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>