Fixed jump threading going to infinite loop.

This patch add code to remove unreachable blocks from function
as they may cause jump threading to stuck in infinite loop.

Differential Revision: http://reviews.llvm.org/D3991



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211103 91177308-0d34-0410-b5e6-96231b3b80d8