[mips] Override TargetLoweringBase::isShuffleMaskLegal.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179433 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Reapply r179420 and r179421.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179434 91177308-0d34-0410-b5e6-96231b3b80d8