[mips][msa] Corrected the definition of the dotp_[su].[hwd] intrinsics

The elements of the operands should be half the width of the elements of
the result.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190505 91177308-0d34-0410-b5e6-96231b3b80d8