Revert my previous change -- the subtraction of two constants was a no-op

Revert my previous change -- the subtraction of two constants was a no-op
before. This is taken care of in the selection DAG pass. In my opinion, this
should be in one place or the other. I.e., it should probably be removed from
the DAG combiner (along with the other arithmetic transformations on constants
that are essentially no-ops).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55889 91177308-0d34-0410-b5e6-96231b3b80d8