[TargetInstrInfo][PowerPC] Remove virtual function that is only called from PPC specific code.

There are two signatures of setSpecialOperandAttr in TargetInstrInfo.
One of them is only called from PPCInstrInfo which has an override
of it.

Remove it from TargetInstrInfo and make it a non-virtual method in
PPCInstrInfo.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D115404