Bugfix for the debug intrinsic handling in InstCombiner:

Since we can't guarantee that the original dbg.declare instrinsic
is removed by LowerDbgDeclare(), we need to make sure that we are
not inserting the same dbg.value intrinsic over and over.
This removes tons of redundant DIEs when compiling optimized code.

rdar://problem/13056109

llvm-svn: 180615