nir/peephole_ffma: Be less agressive about fusing multiply-adds
nir/peephole_ffma: Be less agressive about fusing multiply-adds

shader-db results for fragment shaders on Haswell:
total instructions in shared programs: 4395688 -> 4389623 (-0.14%)
instructions in affected programs:     355876 -> 349811 (-1.70%)
helped:                                1455
HURT:                                  14
GAINED:                                5
LOST:                                  0

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler