InstCombine: Remove infinite loop caused by FoldOpIntoPhi

FoldOpIntoPhi could create an infinite loop if the PHI could potentially
reach a BB it was considering inserting instructions into.  The
instructions it would insert would eventually lead to other combines
firing which would, again, lead to FoldOpIntoPhi firing.

The solution is to handicap FoldOpIntoPhi so that it doesn't attempt to
insert instructions that the PHI might reach.

This fixes PR21377.

llvm-svn: 221187