Improve error handling and message of lowering helper functions. (#9722)

This PR improves the error handling and error message of some lowering
helper functions found in `helpers.{h,cpp}` C++ files.

**Key Changes:**

- Created `Safe*` versions of the functions:
    - `Flatten`
    - `DynamicReshape`
    - `GetDynamicReshapeInfo`
- Created `CheckAtMostOneDynamicDimension()` function, a version of
`GetDynamicDimension` with improved error handling and messages
- Removed the `shape` parameter from `SafeFlatten()`
    - It should be called separately
- Add missing includes ([IWYU ref][1])

[1]:
https://google.github.io/styleguide/cppguide.html#Include_What_You_Use

Author: ysiraichi