Fix uninitialized access in MachineBlockPlacement.

[sammccall]

Summary

Currently PreferredLoopExit is set only in buildLoopChains, which is
never called if there are no MachineLoops.

MSan is currently broken by this:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/145/steps/check-llvm%20msan/logs/stdio

This is a naive fix to get things green again. iteratee: you may have a better fix.

This change will also mean PreferredLoopExit will not carry over if
buildCFGChains() is called a second time in runOnMachineFunction, this
appears to be the right thing.

llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp

for (;;) {
       BB = NextBB;
     }
   }
 
   // Turned on with OutlineOptionalBranches option
   collectMustExecuteBBs();
 
   // Build any loop-based chains.
+  PreferredLoopExit = nullptr;
   for (MachineLoop *L : *MLI)
     buildLoopChains(*L);
 
   assert(BlockWorkList.empty());
   assert(EHPadWorkList.empty());
 
   SmallPtrSet<BlockChain *, 4> UpdatedPreds;
   for (MachineBasicBlock &MBB : *F)