[AArch64][TTI][NFC] Overload method 'getVectorInstrCost' to provide vector...

[AArch64][TTI][NFC] Overload method 'getVectorInstrCost' to provide vector instruction itself, as a context information for cost estimation.

1) Overloaded (instruction-based) method is a wrapper around the current (opcode-based) method.
2) This patch also changes a few callsites (VectorCombine.cpp,
   SLPVectorizer.cpp, CodeGenPrepare.cpp) to call the overloaded method.
3) This is a split of D128302.

Differential Revision: https://reviews.llvm.org/D131114