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/verifier.rs

@@ -251,6 +251,7 @@ impl<'a> Verifier<'a> {
             &InsertLane { .. } |
             &ExtractLane { .. } |
             &IntCompare { .. } |
+            &IntCompareImm { .. } |
             &FloatCompare { .. } => {}
         }
 
@@ -682,4 +683,4 @@ mod tests {
         let verifier = Verifier::new(&func);
         assert_err_with_msg!(verifier.run(), "instruction format");
     }
-}
+}