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/src/prelude.isle

@@ -397,9 +397,6 @@
 (decl emit (MInst) Unit)
 (extern constructor emit emit)
 
-(decl emit_safepoint (MInst) Unit)
-(extern constructor emit_safepoint emit_safepoint)
-
 ;;;; Helpers for Side-Effectful Instructions Without Results ;;;;;;;;;;;;;;;;;;;
 
 (type SideEffectNoResult (enum (Inst (inst MInst))))
@@ -411,12 +408,6 @@
       (let ((_ Unit (emit inst)))
         (output_none)))
 
-;; Similarly, but emit the side-effectful instruction as a safepoint.
-(decl safepoint (SideEffectNoResult) InstOutput)
-(rule (safepoint (SideEffectNoResult.Inst inst))
-      (let ((_ Unit (emit_safepoint inst)))
-        (output_none)))
-
 ;;;; Helpers for Working with Flags ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; Newtype wrapper around `MInst` for instructions that are used for their