cranelift: investigate using union find when resolving value aliases

[fitzgen]

A not-insignificant portion of compilation time is spent in lowering, and when you start expanding the call tree, a lot of that time semi-surprisingly ends up being in value_def. value_def ends up doing repeated alias resolution, my theory is that we are repeatedly doing overlapping alias resolutions. We should investigate using union find to incrementally dedupe this repeated work.

[bjorn3]

The cache for this would need to be cleared when changing the definition of any alias, right?