Don't use invalidated iterators in FlattenCFGPass

Summary:
FlattenCFG may erase unnecessary blocks, which also invalidates iterators to those erased blocks.
Before this patch, `iterativelyFlattenCFG` could try to increment a BB iterator after that BB has been removed and crash.

This patch makes FlattenCFGPass use `WeakVH` to skip over erased blocks.

Reviewers: dblaikie, tstellar, davide, sanjoy, asbirlea, grosser

Reviewed By: asbirlea

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67672

llvm-svn: 372347