[LoopPred] Fix a blatantly obvious bug in r358684

The bug is that I didn't check whether the operand of the invariant_loads were themselves invariant.  I don't know how this got missed in the patch and review.  I even had an unreduced test case locally, and I remember handling this case, but I must have lost it in one of the rebases.  Oops.

llvm-svn: 358688