[LoopSimplifyCFG] Fix inconsistency in blocks in loop markup

2nd part of D57095 with the same reason, just in another place. We never
fold branches that are not immediately in the current loop, but this check
is missing in `IsEdgeLive` As result, it may think that the edge in subloop is
dead while it's live. It's a pessimization in the current stance.

Differential Revision: https://reviews.llvm.org/D57147
Reviewed By: rupprecht	

llvm-svn: 352170