Left shift operation between int and taichi field element is not supported in taichi scope

[BillXu2000]

Describe the bug

I want to use 1 << a[1] in taichi scope, where a is a taichi field, but a TypeError is raised.

To Reproduce

Run the code below.

import taichi as ti
ti.init(debug = True)
a = ti.field(dtype = ti.i32, shape = 3)

@ti.kernel
def fun():
    print(1 << a[1])

a[1] = 2
#print(1 << a[1]) #this line works properly
fun()

Log/Screenshots

bx2k@bx2k-MagicBook:/mnt/c/Users/BillXu2000/Desktop$ cmd.exe /c python3 test.py
[Taichi] mode=release
[Taichi] version 0.7.0, llvm 10.0.0, commit 0298bca3, win, python 3.8.6
[Taichi] Starting on arch=x64
[Taichi] materializing...
Traceback (most recent call last):
  File "test.py", line 11, in <module>
    fun()
  File "C:\Users\BillXu2000\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-pa
ckages\Python38\site-packages\taichi\lang\kernel.py", line 574, in wrapped
    return primal(*args, **kwargs)
  File "C:\Users\BillXu2000\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-pa
ckages\Python38\site-packages\taichi\lang\kernel.py", line 501, in __call__
    self.materialize(key=key, args=args, arg_features=arg_features)
  File "C:\Users\BillXu2000\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-pa
ckages\Python38\site-packages\taichi\lang\kernel.py", line 370, in materialize
    taichi_kernel = taichi_kernel.define(taichi_ast_generator)
  File "C:\Users\BillXu2000\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.8_qbz5n2kfra8p0\LocalCache\local-pa
ckages\Python38\site-packages\taichi\lang\kernel.py", line 367, in taichi_ast_generator
    compiled()
  File "test.py", line 7, in fun
    print(1 << a[1])
TypeError: unsupported operand type(s) for <<: 'int' and 'Expr'