This patch introduces a codegen-only instruction currently named br_unless,
which makes it convenient to implement ReverseBranchCondition and re-enable
the MachineBlockPlacement pass. Then in a late pass, it lowers br_unless
back into br_if.
Differential Revision: http://reviews.llvm.org/D14995
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254826 91177308-0d34-0410-b5e6-96231b3b80d8
