[X86] Remove unreachable code in X86TargetTransformInfo.cpp

In Function getVectorInstrCost, situation Opcode == Instruction::ExtractElement
and Opcode == Instruction::InsertElement are all handled in the first 2 if-statements,
So we have no chance for the code in line 4401.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D145908