Fix uninitialized access in MachineBlockPlacement.

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.

Reviewers: bkramer, iteratee, echristo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26069

llvm-svn: 285757