vc4: Add quick algebraic optimization for clamping of unpacked values.

vc4: Add quick algebraic optimization for clamping of unpacked values.

GL likes to saturate your incoming color, but if that color's coming from
unpacking from unorms, there's no point.  Ideally we'd have a range
propagation pass that cleans these up in NIR, but that doesn't seem to be
going to land soon.  It seems like we could do a one-off optimization in
nir_opt_algebraic, except that doesn't want to operate on expressions
involving unpack_unorm_4x8, since it's sized.

total instructions in shared programs: 87879 -> 87761 (-0.13%)
instructions in affected programs:     6044 -> 5926 (-1.95%)
total estimated cycles in shared programs: 349457 -> 349252 (-0.06%)
estimated cycles in affected programs:     6172 -> 5967 (-3.32%)

No SSPD on openarena (which had the biggest gains, in its VS/CSes), n=15.