nir/gather_info: Handle discard_if
nir/gather_info: Handle discard_if

Reviewed-by: Nanley Chery <d78cd5d33e98a5581566f188959f731bd0b1c0fd@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler