Cranelift: develop an alias analysis

[cfallin]

We should build a simple alias analysis over CLIF to determine when loads and stores are referring to overlapping (may-alias) or the same (must-alias) data, and allow optimizations as appropriate (redundant-load elimination, store-to-load forwarding, dead-store elimination, code motion in general in LICM or lowering).

It is likely we will want to build a bitset-style analysis, where we have a small, finite set of disjoint divisions of the heap, and every load or store is marked as potentially accessing one or more.

For example, one could imagine a set of four bits for a Wasm frontend: accesses to any heap, accesses to any table, accesses to any global, or everything else (and explicitly not one of the above).

Given these bits for every load/store, we then have a "vector color" of sorts, in the sense that the current instruction coloring scheme could be seen to have a color per disjoint segment of world-state. We don't actually necessarily want to compute these vectors of colors, but we want to be able to answer questions using them.