[ir] ti.func parameter types are not casted

[bobcao3]

Repro:

import taichi as ti

ti.init()

field = ti.field(dtype=ti.f32, shape=(50))
index = ti.field(dtype=ti.i16, shape=())

@ti.func
def get_data(idx : ti.i32):
    return field[idx]

@ti.kernel
def run() -> ti.f32:
    return get_data(index[None])

run()

This emits warning:

[W 05/06/23 17:30:22.109 12880] [type_check.cpp:taichi::lang::TypeCheck::visit@154] [$11] Field index 0 not int32, casting into int32 implicitly
File "C:\Users\bobca\source\repos\meshy\repro0.py", line 10, in get_data:
    return field[idx]
           ^^^^^^^^^^

I'd expect if I declare function parameter as ti.i32, when I'm calling the function argument should be passed by value and casted to i32, it seems it stayed as i16.