Allow BB duplication threshold to be adjusted through JumpThreading's ctor
    
- BB duplication may not be desired on targets where there is no or small
  branch penalty and code duplication needs restrict control.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218375 91177308-0d34-0410-b5e6-96231b3b80d8