nir: Add opcodes for saturated vector math.

nir: Add opcodes for saturated vector math.

This corresponds to instructions used on vc4 for its blending inside of
shaders.  I've seen these opcodes on other architectures before, but I
think it's the first time these are needed in Mesa.

v2: Rename to 'u' instead of 'i', since they're all 'u'norm (from review
    by jekstrand)