[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