AstGen: fix `while` and `for` with unreachable bodies
Companion commit to 61a53a587558ff1fe1b0ec98bb424022885edccf.
This commit also moves over a bunch of behavior test cases to the
passing-for-stage2 section.
c4df9bf56fAndrew Kelley committed on 10/3/2021, 3:15:03 AMparent61a53a5