[RISCV][TTI] Remove SK_Select from manual splitting in getShuffleCost

We have general splitting logic for this kind just below, which to my
knowledge is both correct and precise.  Given no test changes, either
a) the adhoc logic works out the same, or b) we have no coverage. I
did not investigate which.

Author: preames

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -585,7 +585,6 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
       case TTI::SK_PermuteSingleSrc:
       case TTI::SK_Transpose:
       case TTI::SK_PermuteTwoSrc:
-      case TTI::SK_Select:
         return true;
       }
     };