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/isa/unwind/systemv.rs

@@ -1,8 +1,9 @@
 //! System V ABI unwind information.
 
-use crate::binemit::CodeOffset;
 use crate::isa::unwind::UnwindInst;
-use crate::result::{CodegenError, CodegenResult};
+use crate::machinst::Reg;
+use crate::result::CodegenResult;
+use crate::{binemit::CodeOffset, CodegenError};
 use alloc::vec::Vec;
 use gimli::write::{Address, FrameDescriptionEntry};
 
@@ -160,7 +161,7 @@ pub struct UnwindInfo {
     len: u32,
 }
 
-pub(crate) fn create_unwind_info_from_insts<MR: RegisterMapper<regalloc::Reg>>(
+pub(crate) fn create_unwind_info_from_insts<MR: RegisterMapper<Reg>>(
     insts: &[(CodeOffset, UnwindInst)],
     code_len: usize,
     mr: &MR,
@@ -237,7 +238,7 @@ pub(crate) fn create_unwind_info_from_insts<MR: RegisterMapper<regalloc::Reg>>(
                 reg,
             } => {
                 let reg = mr
-                    .map(reg.to_reg())
+                    .map(reg.into())
                     .map_err(|e| CodegenError::RegisterMappingError(e))?;
                 let off = (clobber_offset as i32) - (clobber_offset_to_cfa as i32);
                 instructions.push((instruction_offset, CallFrameInstruction::Offset(reg, off)));