Switch Cranelift over to regalloc2. (#3989)
This PR switches Cranelift over to the new register allocator, regalloc2. See [this document](https://gist.github.com/cfallin/08553421a91f150254fe878f67301801) for a summary of the design changes. This switchover has implications for core VCode/MachInst types and the lowering pass. Overall, this change brings improvements to both compile time and speed of generated code (runtime), as reported in #3942: ``` Benchmark Compilation (wallclock) Execution (wallclock) blake3-scalar 25% faster 28% faster blake3-simd no diff no diff meshoptimizer 19% faster 17% faster pulldown-cmark 17% faster no diff bz2 15% faster no diff SpiderMonkey, 21% faster 2% faster fib(30) clang.wasm 42% faster N/A ```
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