ARM64 backend, part 1 / 11: misc changes to existing code.

- Add a `simple_legalize()` function that invokes a predetermined set of
  legalizations, without depending on the details of the current
  backend design. This will be used by the new backend pipeline.

- Separate out `has_side_effect()` from the DCE pass. This will be used
  by the new backends' lowering code.

- Add documentation for the `Arm64Call` relocation type.

cranelift/codegen/src/legalizer/mod.rs

@@ -196,6 +196,55 @@ pub fn legalize_function(func: &mut ir::Function, cfg: &mut ControlFlowGraph, is
     }
 }
 
+/// Perform a simple legalization by expansion of the function, without
+/// platform-specific transforms.
+pub fn simple_legalize(func: &mut ir::Function, cfg: &mut ControlFlowGraph, isa: &dyn TargetIsa) {
+    let mut pos = FuncCursor::new(func);
+    let func_begin = pos.position();
+    pos.set_position(func_begin);
+    while let Some(_block) = pos.next_block() {
+        let mut prev_pos = pos.position();
+        while let Some(inst) = pos.next_inst() {
+            let expanded = match pos.func.dfg[inst].opcode() {
+                ir::Opcode::BrIcmp
+                | ir::Opcode::GlobalValue
+                | ir::Opcode::HeapAddr
+                | ir::Opcode::StackLoad
+                | ir::Opcode::StackStore
+                | ir::Opcode::TableAddr
+                | ir::Opcode::Trapnz
+                | ir::Opcode::Trapz
+                | ir::Opcode::BandImm
+                | ir::Opcode::BorImm
+                | ir::Opcode::BxorImm
+                | ir::Opcode::IaddImm
+                | ir::Opcode::IfcmpImm
+                | ir::Opcode::ImulImm
+                | ir::Opcode::IrsubImm
+                | ir::Opcode::IshlImm
+                | ir::Opcode::RotlImm
+                | ir::Opcode::RotrImm
+                | ir::Opcode::SdivImm
+                | ir::Opcode::SremImm
+                | ir::Opcode::SshrImm
+                | ir::Opcode::UdivImm
+                | ir::Opcode::UremImm
+                | ir::Opcode::UshrImm
+                | ir::Opcode::IcmpImm => expand(inst, &mut pos.func, cfg, isa),
+                _ => false,
+            };
+
+            if expanded {
+                // Legalization implementations require fixpoint loop
+                // here. TODO: fix this.
+                pos.set_position(prev_pos);
+            } else {
+                prev_pos = pos.position();
+            }
+        }
+    }
+}
+
 // Include legalization patterns that were generated by `gen_legalizer.rs` from the
 // `TransformGroup` in `cranelift-codegen/meta/shared/legalize.rs`.
 //