ARM64 backend, part 8 / 11: integration.

This patch ties together the new backend infrastructure with the
existing Cranelift codegen APIs.

With all patches in this series up to this patch applied, the ARM64
compiler is now functional and can be used. Two uses of this
functionality -- filecheck-based tests and integration into wasmtime --
will come in subsequent patches.

cranelift/codegen/src/ir/function.rs

@@ -3,6 +3,8 @@
 //! The `Function` struct defined in this module owns all of its basic blocks and
 //! instructions.
 
+#![allow(unused_imports)]
+
 use crate::binemit::CodeOffset;
 use crate::entity::{PrimaryMap, SecondaryMap};
 use crate::ir;
@@ -17,6 +19,7 @@ use crate::isa::{CallConv, EncInfo, Encoding, Legalize, TargetIsa};
 use crate::regalloc::{EntryRegDiversions, RegDiversions};
 use crate::value_label::ValueLabelsRanges;
 use crate::write::write_function;
+use alloc::boxed::Box;
 use core::fmt;
 
 /// A function.
@@ -238,13 +241,21 @@ impl Function {
 
     /// Wrapper around `encode` which assigns `inst` the resulting encoding.
     pub fn update_encoding(&mut self, inst: ir::Inst, isa: &dyn TargetIsa) -> Result<(), Legalize> {
-        self.encode(inst, isa).map(|e| self.encodings[inst] = e)
+        if isa.get_mach_backend().is_some() {
+            Ok(())
+        } else {
+            self.encode(inst, isa).map(|e| self.encodings[inst] = e)
+        }
     }
 
     /// Wrapper around `TargetIsa::encode` for encoding an existing instruction
     /// in the `Function`.
     pub fn encode(&self, inst: ir::Inst, isa: &dyn TargetIsa) -> Result<Encoding, Legalize> {
-        isa.encode(&self, &self.dfg[inst], self.dfg.ctrl_typevar(inst))
+        if isa.get_mach_backend().is_some() {
+            Ok(Encoding::new(0, 0))
+        } else {
+            isa.encode(&self, &self.dfg[inst], self.dfg.ctrl_typevar(inst))
+        }
     }
 
     /// Starts collection of debug information.