nir: Add a SSA type gathering pass

This new pass (which isn't even compile-tested) attempts to determine the ALU type of all the SSA values in a function impl. It takes a greedy approach and assigns intness or floatness to everything it thinks can possibly contain an int or a float. Some values will be labled as both int and float and some will be labled as neither and it is up to the caller to decide what to do with this information. However, for a "nice" shader where the original source contained no bit-casts and no implicit bit-casts were introduced by optimizations, there shouldn't be any overlap in the two sets save for the odd CSEd zero constant.