[mips][msa] Removed ldx.[bhwd] and stx.[bhwd].

These were present in a previous version of the MSA spec but are not
present in the published version. There is no hardware that uses these
instructions.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192888 91177308-0d34-0410-b5e6-96231b3b80d8