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
```

cranelift/codegen/Cargo.toml

@@ -23,7 +23,7 @@ serde = { version = "1.0.94", features = ["derive"], optional = true }
 bincode = { version = "1.2.1", optional = true }
 gimli = { version = "0.26.0", default-features = false, features = ["write"], optional = true }
 smallvec = { version = "1.6.1" }
-regalloc = "0.0.34"
+regalloc2 = { version = "0.1.1", features = ["checker"] }
 souper-ir = { version = "2.1.0", optional = true }
 # It is a goal of the cranelift-codegen crate to have minimal external dependencies.
 # Please don't add any unless they are essential to the task of creating binary
@@ -77,14 +77,10 @@ all-arch = [
 # For dependent crates that want to serialize some parts of cranelift
 enable-serde = [
     "serde",
-    "regalloc/enable-serde",
     "cranelift-entity/enable-serde",
+    "regalloc2/enable-serde",
 ]
 
-# Allow snapshotting regalloc test cases. Useful only to report bad register
-# allocation failures, or for regalloc.rs developers.
-regalloc-snapshot = ["bincode", "regalloc/enable-serde"]
-
 # Enable support for the Souper harvester.
 souper-harvest = ["souper-ir", "souper-ir/stringify"]