Fuzzy CSE matching

This may be captured else where, but there's a general idea of an optimization like "fuzzy-tradeoff sharing".

In traditional compiler passes you CSE (common subexpression eliminate) exact matches in operations. In hardware, however, you could "fuzzy match" and just mask off wires, e.g. imagine a 4 bit adder and a 5 bit adder, you could reuse one unit instead of specializing for both, given they have the same operands.

E.g. an example @grebe just gave: `a + b` vs `a[3:0] + b[3:0]`.

(From there, you could generalize to "different operands", but that requires proof of mutual exclusion if you're going to reuse structural operator resources as in the resource binding problem. There's a simpler version with the same operands that doesn't require exclusivity.)