[TargetLowering] Change getOptimalMemOpType and findOptimalMemOpLowering to take LLVM Context (#147664)
Add LLVM Context to getOptimalMemOpType and findOptimalMemOpLowering. So
that we can use EVT::getVectorVT to generate EVT type in
getOptimalMemOpType.
Related to [#146673](https://github.com/llvm/llvm-project/pull/146673).