If SymInt::maybe_as_int() returns non-empty, then we get an inline
fast path. The philosophy here (as with the previous PR) is to
preserve performance in the "plain old ints" case.
Observed time spent in SymInt functions in computeStorageNBytes to
drop (and not cost shift elsewhere in the function) after this change,
profiling detach() using code similar to the benchmark from #160580
and Linux perf.
Differential Revision: [D81530107](https://our.internmc.facebook.com/intern/diff/D81530107)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/161586
Approved by: https://github.com/ezyang
ghstack dependencies: #161466
b0a3e58dd7