nir/algebraic: add lowering for fsign

The mali utgard pp doesn't support a sign instruction.
In the ARM offline shader compiler, the sign function is implemented
using sub(gt(0.0, a), lt(0.0, a)).
This is a generic optimization, so implement it in the nir level when
lower_fsign is set, alongside the lowering for isign.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/nir.h

@@ -2242,6 +2242,9 @@ typedef struct nir_shader_compiler_options {
    /** enables rules to lower isign to imin+imax */
    bool lower_isign;
 
+   /** enables rules to lower fsign to fsub and flt */
+   bool lower_fsign;
+
    /* Does the native fdot instruction replicate its result for four
     * components?  If so, then opt_algebraic_late will turn all fdotN
     * instructions into fdot_replicatedN instructions.