nir: make lower_clamp_color pass work after lower i/o
nir: make lower_clamp_color pass work after lower i/o

Kinda important to work with tgsi_to_nir, which generates nir which
already has i/o lowered.

Signed-off-by: Rob Clark <6f78cdb10adbe0a7fbbf4098125a51f401c77a5f@freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler