Cranelift: reuse compare results (flags) when possible

Currently, in both the x64 and aarch64 backends, we generate flags "locally" whenever needed. So, for example, the sequence

```
v0 = icmp ...
v1 = select v0, ...
brnz v0, blockN
jmp blockM
```

would generate the compare for `v0` twice.

This is generally a simplification over the old `iflags` approach, which we prefer for some of the reasons described in #3249. In particular, `iflags`-typed values are "weird" (cannot be stored or loaded, only one can be live at a time, cannot be directly observed) and these restrictions complicate other analyses/transforms. The tradeoff of effectively regenerating them on each use has been reasonable so far.

However, locally in cases where we do use the results of a compare more than once, we should be able to share a single compare operation. We might be able to reason about this by building a forward pass that tracks the last generated flags and using this information from within the backend's pattern-matching. For extra credit, we might be able to factor this information into the "unique use" framework, allowing a compare with multiple uses but only one codegen'd occurrence to merge load operations directly on x64.