Clamp negative to zero and clamp positive to allOnesValue are common
operation in image saturation.
Add tests for shifty implementation of clamping, as prepare work for
folding:
and(ashr(subNSW(X, V), ScalarSizeInBits - 1), V) --> V s> 0 ? V : 0;
or(ashr(subNSW(X, V), ScalarSizeInBits - 1), V) --> V s> X ? AllOnesValue : V.