message: Use an imul_imm, I guess
diff: @@ -729,8 +729,7 @@ fn compile_inst(inst: &bril::Instruction, builder: &mut FunctionBuilder, env: &C
  let amt = builder.use_var(env.vars[&args[1]]);
  let size = pointee_bytes(op_type, env.pointer_type);
  - let bytes_val = builder.ins().iconst(ir::types::I64, size as i64);
  - let offset_val = builder.ins().imul(amt, bytes_val);
  + let offset_val = builder.ins().imul_imm(amt, size as i64);
  let res = builder.ins().iadd(orig_ptr, offset_val);
  builder.def_var(env.vars[dest], res);
  