[opt:ra] Add range analysis support for Encode op.

[cdleary]

Takes a whack at range analysis for the encode IR op.

Fast exact case (small input range): If the input’s IntervalSet contains only a small number of concrete values (<= kMaxIterationSize), we enumerate each possible input value, compute encode result exactly, and union the results. This produces an exact output set and is still cheap because the input set is small by construction.

Fallback (large input range): If the input range is large, we avoid enumerating it. Instead, we extract a ternary mask for the input bits (known-0 / known-1 / unknown) from the input interval set, and compute each output bit using the encode op definition ("output bit j is the OR of input bits whose index has bit j set"). If any input bit is known-1 the output bit is known-1; if none are known-1 but at least one is unknown the output bit is unknown, otherwise it’s known-0. We convert that output ternary mask into an IntervalSet (interval_ops::FromTernary), which is conservative but fast.

Fuzzed multiprocess for >15 mins on a many-core machine.