Merge empty landing pads in SimplifyCFG

Merge empty landing pads in SimplifyCFG

This patch tries to merge duplicate landing pads when they branch to a common shared target.

Given IR that looks like this:
lpad1:
  %exn = landingpad {i8*, i32} personality i32 (...)* @__gxx_personality_v0
         cleanup
  br label %shared_resume
lpad2:
  %exn2 = landingpad {i8*, i32} personality i32 (...)* @__gxx_personality_v0
          cleanup
  br label %shared_resume
shared_resume:
  call void @fn()
  ret void
}

We can rewrite the users of both landing pad blocks to use one of them. This will generally allow the shared_resume block to be merged with the common landing pad as well.

Without this change, tail duplication would likely kick in - creating N (2 in this case) copies of the shared_resume basic block.

Differential Revision: http://reviews.llvm.org/D8297

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233125 91177308-0d34-0410-b5e6-96231b3b80d8