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/machinst/abi.rs

@@ -5,7 +5,6 @@ use crate::ir::{Signature, StackSlot};
 use crate::isa::CallConv;
 use crate::machinst::*;
 use crate::settings;
-use regalloc::{Reg, Set, SpillSlot, Writable};
 use smallvec::SmallVec;
 
 /// A small vector of instructions (with some reasonable size); appropriate for
@@ -42,12 +41,6 @@ pub trait ABICallee {
     /// Get the calling convention implemented by this ABI object.
     fn call_conv(&self) -> CallConv;
 
-    /// Get the liveins of the function.
-    fn liveins(&self) -> Set<RealReg>;
-
-    /// Get the liveouts of the function.
-    fn liveouts(&self) -> Set<RealReg>;
-
     /// Number of arguments.
     fn num_args(&self) -> usize;
 
@@ -106,7 +99,7 @@ pub trait ABICallee {
     fn set_num_spillslots(&mut self, slots: usize);
 
     /// Update with the clobbered registers, post-regalloc.
-    fn set_clobbered(&mut self, clobbered: Set<Writable<RealReg>>);
+    fn set_clobbered(&mut self, clobbered: Vec<Writable<RealReg>>);
 
     /// Get the address of a stackslot.
     fn stackslot_addr(&self, slot: StackSlot, offset: u32, into_reg: Writable<Reg>) -> Self::I;