[mips][msa] Removed unsupported dot product instructions (dotp_[su].b)

The dotp_[su].b instructions never existed in any revision of the MSA spec.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190398 91177308-0d34-0410-b5e6-96231b3b80d8