Revert "[SLP]Use source vector type as the original vector type instead of"

This reverts commit 9a99944d.

Due to test suite failures on all our SVE buildbots e.g.:
https://lab.llvm.org/buildbot/#/builders/184/builds/7375

clang: ../llvm/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp:3565:
InstructionCost llvm::AArch64TTIImpl::getShuffleCost(TTI::ShuffleKind,
VectorType *, ArrayRef<int>, TTI::TargetCostKind, int, VectorType *,
ArrayRef<const Value *>): Assertion `Mask.size() == TpNumElts && "Expected Mask and Tp size to match!"' failed.