Reimplement the loopsimplify code which deletes edges from unreachable

Reimplement the loopsimplify code which deletes edges from unreachable
blocks that target loop blocks.

Before, the code was run once per loop, and depended on the number of
predecessors each block in the loop had.  Unfortunately, scanning preds can
be really slow when huge numbers of phis exist or when phis with huge numbers
of inputs exist.

Now, the code is run once per function and scans successors instead of preds,
which is far faster.  In addition, the new code is simpler and is goto free,
woo.

This change speeds up a nasty testcase Duraid provided me from taking hours to
taking ~72s with a debug build.  The functionality this implements is already
tested in the testsuite as Transforms/CodeExtractor/2004-03-13-LoopExtractorCrash.ll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29644 91177308-0d34-0410-b5e6-96231b3b80d8