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
```

Cargo.lock

@@ -529,7 +529,7 @@ dependencies = [
  "hashbrown 0.9.1",
  "log",
  "miette",
- "regalloc",
+ "regalloc2",
  "serde",
  "smallvec",
  "souper-ir",
@@ -1219,6 +1219,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "fxhash"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c31b6d751ae2c7f11320402d34e41349dd1016f8d5d45e48c4312bc8625af50c"
+dependencies = [
+ "byteorder",
+]
+
 [[package]]
 name = "generic-array"
 version = "0.14.5"
@@ -2393,14 +2402,15 @@ dependencies = [
 ]
 
 [[package]]
-name = "regalloc"
-version = "0.0.34"
+name = "regalloc2"
+version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "62446b1d3ebf980bdc68837700af1d77b37bc430e524bf95319c6eada2a4cc02"
+checksum = "3dd122b168f0046afcde717e002cdf76c9c87f829ae99dd12a02a0dcf7cc68f1"
 dependencies = [
+ "fxhash",
  "log",
- "rustc-hash",
  "serde",
+ "slice-group-by",
  "smallvec",
 ]
 
@@ -2653,6 +2663,12 @@ dependencies = [
  "rand_core 0.6.3",
 ]
 
+[[package]]
+name = "slice-group-by"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "03b634d87b960ab1a38c4fe143b508576f075e7c978bfad18217645ebfdfa2ec"
+
 [[package]]
 name = "smallvec"
 version = "1.8.0"