[SimplifyCFG] avoid sinking insts within an infinite-loop

[SimplifyCFG] avoid sinking insts within an infinite-loop

The test is reduced from a C source example in:
https://llvm.org/PR49541

It's possible that the test could be reduced further or
the predicate generalized further, but it seems to require
a few ingredients (including the "late" SimplifyCFG options
on the RUN line) to fall into the infinite-loop trap.

(cherry picked from commit bd197ed0a57a82187ed3c6265ca811d412acfaef)