Fix SCTC bug when two pred/succ BB are in a loop.

Summary: It's possible that two basic blocks being conidered for SCTC are in a loop in the CFG.  In this case a block that is both a predecessor and a successor may have been processed and marked invalid by a previous iteration of the SCTC loop. We should skip rewriting in this case.

(cherry picked from FBD5886721)