as the current DSE, but it only a linear scan over each block, rather than quadratic. Eventually
(once it has been improved somewhat), this will replace the current DSE.
NOTE: This has not yet been extensively tested.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@38517 91177308-0d34-0410-b5e6-96231b3b80d8