[GlobalISel] Fix wrong invocation of `getParamStackAlign` (NFC)

The function template `CallLowering::setArgFlags` is invoked both
for arguments and return values. In the latter case, it calls
`getParamStackAlign` with argument index `~0u`. Nothing wrong
happens now, as the argument is safely incremented back to 0
inside `getParamStackAlign` (the type is `unsigned`), but in
principle it's fragile and may become incorrect.

Differential Revision: https://reviews.llvm.org/D102004