nv50/ir: Decouple DataArray from the dictionary that maps locations to values.

nv50/ir: Decouple DataArray from the dictionary that maps locations to values.

The point is to keep an independent dictionary for each function.

The array that was being used as dictionary has been converted into a
"bimap" for two different reasons: first, because having an almost
empty instance of an array with as many entries as registers there are
in the program, once for every function, would be wasteful, and
second, because we want to be able to map Value pointers back to
locations at some point.