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/compile.rs

@@ -14,6 +14,7 @@ pub fn compile<B: LowerBackend + MachBackend>(
     f: &Function,
     b: &B,
     abi: Box<dyn ABICallee<I = B::MInst>>,
+    emit_info: <B::MInst as MachInstEmit>::Info,
 ) -> CodegenResult<VCode<B::MInst>>
 where
     B::MInst: PrettyPrint,
@@ -21,7 +22,7 @@ where
     // Compute lowered block order.
     let block_order = BlockLoweringOrder::new(f);
     // Build the lowering context.
-    let lower = Lower::new(f, abi, block_order)?;
+    let lower = Lower::new(f, abi, emit_info, block_order)?;
     // Lower the IR.
     let (mut vcode, stack_map_request_info) = {
         let _tt = timing::vcode_lower();