Don't rely on undefined left shift behavior in assert in PyStackRef_TagInt

[DinoV]

Bug report

Bug description:

This assert https://github.com/python/cpython/blob/main/Include/internal/pycore_stackref.h#L406 contains a left shift of a potentially negative value which is undefined behavior. The non-assert line does a uintptr_t cast to avoid the undefined behavior. Replicate that in the assert and then cast the final value back to intptr_t.

This can show up when running with various sanitizers enabled.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-140868: Don't rely on undefined left shift behavior in assert #140869