If we are extracting a block that has multiple successors that are the same

If we are extracting a block that has multiple successors that are the same
block (common in a switch), make sure to remove extra edges in successor
blocks.  This fixes CodeExtractor/2004-08-12-BlockExtractPHI.ll and should
be pulled into LLVM 1.3 (though the regression test need not be, as that
would require pulling in the LoopExtract.cpp changes).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15717 91177308-0d34-0410-b5e6-96231b3b80d8