ARM64 backend, part 3 / 11: MachInst infrastructure.

This patch adds the MachInst, or Machine Instruction, infrastructure.
This is the machine-independent portion of the new backend design. It
contains the implementation of the "vcode" (virtual-registerized code)
container, the top-level lowering algorithm and compilation pipeline,
and the trait definitions that the machine backends will fill in.

This backend infrastructure is included in the compilation of the
`codegen` crate, but it is not yet tied into the public APIs; that patch
will come last, after all the other pieces are filled in.

This patch contains code written by Julian Seward <jseward@acm.org> and
Benjamin Bouvier <public@benj.me>, originally developed on a side-branch
before rebasing and condensing into this patch series. See the `arm64`
branch at `https://github.com/cfallin/wasmtime` for original development
history.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Benjamin Bouvier <public@benj.me>

cranelift/codegen/src/isa/mod.rs

@@ -55,9 +55,10 @@ pub use crate::isa::stack::{StackBase, StackBaseMask, StackRef};
 use crate::binemit;
 use crate::flowgraph;
 use crate::ir;
-use crate::isa::enc_tables::Encodings;
+pub use crate::isa::enc_tables::Encodings;
 #[cfg(feature = "unwind")]
 use crate::isa::fde::RegisterMappingError;
+use crate::machinst::MachBackend;
 use crate::regalloc;
 use crate::result::CodegenResult;
 use crate::settings;
@@ -400,6 +401,11 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     ) {
         // No-op by default
     }
+
+    /// Get the new-style MachBackend, if this is an adapter around one.
+    fn get_mach_backend(&self) -> Option<&dyn MachBackend> {
+        None
+    }
 }
 
 impl Debug for &dyn TargetIsa {