Fix CVE-2020-1914 by using NEXTINST for SaveGeneratorLong

Summary:
If `SaveGeneratorLong` was emitted, it would accidentally jump to the
wrong next instruction, based on how long SaveGenerator was.

Make a callout function to handle the common case, and handle the dispatch
within each case of the interpreter loop.

Fixes CVE-2020-1914

Reviewed By: neildhar

Differential Revision: D24024242

fbshipit-source-id: 3bcb88daa740f0d50e91771a49eb212551ce8bd8