merge-locals pass (#1334)

This optimizes the situation described in #1331. Namely, when x is copied into y, then on subsequent gets of x we could use y instead, and vice versa, as their value is equal. Specifically, this seems to get rid of the definite overlap in the live ranges of x and y, as removing it allows coalesce-locals to merge them. The pass therefore does nothing if the live range of y ends there anyhow.

The danger here is that we may extend the live range so that it causes more conflicts with other things, so this is a heuristic, but I've tested it on every codebase I can find and it always produces a net win, even on one I saw a 0.4% reduction of code size, which surprised me.

This is a fairly slow pass, because it uses LocalGraph which isn't much optimized. This PR includes a minor optimization for it, but we should rewrite it. Meanwhile this is just enabled in -O3 and -Oz.

This PR also includes some fuzzing improvements, to better test stuff like this.

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