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/binemit/mod.rs

@@ -54,7 +54,9 @@ pub enum Reloc {
     X86GOTPCRel4,
     /// Arm32 call target
     Arm32Call,
-    /// Arm64 call target
+    /// Arm64 call target. Encoded as bottom 26 bits of instruction. This
+    /// value is sign-extended, multiplied by 4, and added to the PC of
+    /// the call instruction to form the destination address.
     Arm64Call,
     /// RISC-V call target
     RiscvCall,