Revert "[SCEV] Do not fold dominated SCEVUnknown into AddRecExpr start"

This reverts commit r303730 because it broke all the buildbots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303747 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed