[SCCP] Remove duplicate code

SCCP has code identical to changeToUnreachable's behavior, switch it
over to just call changeToUnreachable.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258654 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed