Add an icmp_imm instruction.

Compare a scalar integer to an immediate constant. Both Intel and RISC-V
ISAs have this operation.

This requires the addition of a new IntCompareImm instruction format.

lib/cretonne/src/write.rs

@@ -260,6 +260,7 @@ pub fn write_operands(w: &mut Write, dfg: &DataFlowGraph, inst: Inst) -> Result
         InsertLane { lane, args, .. } => write!(w, " {}, {}, {}", args[0], lane, args[1]),
         ExtractLane { lane, arg, .. } => write!(w, " {}, {}", arg, lane),
         IntCompare { cond, args, .. } => write!(w, " {}, {}, {}", cond, args[0], args[1]),
+        IntCompareImm { cond, arg, imm, .. } => write!(w, " {}, {}, {}", cond, arg, imm),
         FloatCompare { cond, args, .. } => write!(w, " {}, {}, {}", cond, args[0], args[1]),
         Jump { destination, ref args, .. } => {
             if args.is_empty() {