[mips] Removed two-operand alias for sllv, sr[al]v, rotrv, dsllv, dsr[al]v, and drotrv

[mips] Removed two-operand alias for sllv, sr[al]v, rotrv, dsllv, dsr[al]v, and drotrv

GAS doesn't actually accept these particular cases.

The mnemonic without the trailing 'v' still supports two-operand aliases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207740 91177308-0d34-0410-b5e6-96231b3b80d8