machinst: allow passing constant information to the instruction emitter;

A new associated type Info is added to MachInstEmit, which is the
immutable counterpart to State. It can't easily be constructed from an
ABICallee, since it would require adding an associated type to the
latter, and making so leaks the associated type in a lot of places in
the code base and makes the code harder to read. Instead, the EmitInfo
state can simply be passed to the `Vcode::emit` function directly.

cranelift/codegen/src/machinst/mod.rs

@@ -275,12 +275,21 @@ pub enum MachTerminator<'a> {
 pub trait MachInstEmit: MachInst {
     /// Persistent state carried across `emit` invocations.
     type State: MachInstEmitState<Self>;
+    /// Constant information used in `emit` invocations.
+    type Info: MachInstEmitInfo;
     /// Emit the instruction.
-    fn emit(&self, code: &mut MachBuffer<Self>, flags: &Flags, state: &mut Self::State);
+    fn emit(&self, code: &mut MachBuffer<Self>, info: &Self::Info, state: &mut Self::State);
     /// Pretty-print the instruction.
     fn pretty_print(&self, mb_rru: Option<&RealRegUniverse>, state: &mut Self::State) -> String;
 }
 
+/// Constant information used to emit an instruction.
+pub trait MachInstEmitInfo {
+    /// Return the target-independent settings used for the compilation of this
+    /// particular function.
+    fn flags(&self) -> &Flags;
+}
+
 /// A trait describing the emission state carried between MachInsts when
 /// emitting a function body.
 pub trait MachInstEmitState<I: MachInst>: Default + Clone + Debug {