[MIPS] Add aliases for sync instruction used by Octeon CPU

This commit adds aliases for the sync instruction (synciobdma,
syncs, syncw, syncws) which are used by the Octeon CPU.

Reviewed by D. Sanders

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217477 91177308-0d34-0410-b5e6-96231b3b80d8