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).

llvm-svn: 15717