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/s390x/inst/imms.rs

@@ -1,6 +1,6 @@
 //! S390x ISA definitions: immediate constants.
 
-use regalloc::{PrettyPrint, RealRegUniverse};
+use crate::machinst::{AllocationConsumer, PrettyPrint};
 use std::string::String;
 
 /// An unsigned 12-bit immediate.
@@ -207,25 +207,25 @@ impl UImm32Shifted {
 }
 
 impl PrettyPrint for UImm12 {
-    fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
+    fn pretty_print(&self, _: u8, _: &mut AllocationConsumer<'_>) -> String {
         format!("{}", self.value)
     }
 }
 
 impl PrettyPrint for SImm20 {
-    fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
+    fn pretty_print(&self, _: u8, _: &mut AllocationConsumer<'_>) -> String {
         format!("{}", self.value)
     }
 }
 
 impl PrettyPrint for UImm16Shifted {
-    fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
+    fn pretty_print(&self, _: u8, _: &mut AllocationConsumer<'_>) -> String {
         format!("{}", self.bits)
     }
 }
 
 impl PrettyPrint for UImm32Shifted {
-    fn show_rru(&self, _mb_rru: Option<&RealRegUniverse>) -> String {
+    fn pretty_print(&self, _: u8, _: &mut AllocationConsumer<'_>) -> String {
         format!("{}", self.bits)
     }
 }