[MBP] Remove an invalid assert.

The patch that this assert comes with is fixing a bug in MBP. The assert is
invalid however.

Thanks to @sergey.k.okunev for finding this

Currently this fails SPECCPU2006 LTO. I will add a test case when I do more
investigation and have one.

llvm-svn: 315032

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -2240,12 +2240,6 @@ void MachineBlockPlacement::buildLoopChains(const MachineLoop &L) {
   if (!RotateLoopWithProfile && LoopTop == L.getHeader())
     PreferredLoopExit = findBestLoopExit(L, LoopBlockSet);
 
-  // Make sure PreferredLoopExit actually exits the current loop.
-  if (PreferredLoopExit) {
-    assert(L.isLoopExiting(PreferredLoopExit) &&
-           "not an exiting block of current loop");
-  }
-
   BlockChain &LoopChain = *BlockToChain[LoopTop];
 
   // FIXME: This is a really lame way of walking the chains in the loop: we