[mips][mips64r6] Correct the encoding of dmuh, dmuhu, dmul, and dmulu.

We have detected a documentation bug in the encoding tables of the released
MIPS64r6 specification that has resulted in the wrong encodings being used for
these instructions in LLVM. This commit corrects them.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212330 91177308-0d34-0410-b5e6-96231b3b80d8