[PATCH] Merge empty landing pads in SimplifyCFG

[Philip Reames]

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.  

There are two subtitles with the choice of blocks to merge.  From the comments:
/// We specifically choose to not worry about merging non-empty blocks
/// here.  That is a PRE/scheduling problem and is best solved elsewhere.  In
/// practice, the optimizer produces empty landing pad blocks quite frequently
/// when dealing with exception dense code.  (see: instcombine, gvn, if-else
/// sinking in this file)
///
/// This is primarily a code size optimization.  We need to avoid performing
/// any transform which might inhibit optimization (such as our ability to
/// specialize a particular handler via tail commoning).  We do this by not
/// merging any blocks which require us to introduce a phi.  Since the same
/// values are flowing through both blocks, we don't loose any ability to
/// specialize.  If anything, we make such specialization more likely.

http://reviews.llvm.org/D8297

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/duplicate-landingpad.ll

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