Revert "[SLP]Improve tryToGatherExtractElements by using per-register analysis."

This causes asserts:

  llvm-project/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:10082:
  Value *llvm::slpvectorizer::BoUpSLP::ShuffleInstructionBuilder::adjustExtracts(
    const TreeEntry *, MutableArrayRef<int>, unsigned int, bool &):
  Assertion `Part == 0 && "Expected firs part."' failed.

See comment on the code review.

> Currently tryToGatherExtractElements function analyzes the whole vector,
> regrdless number of actual registers, used in this vector. It may
> prevent some optimizations, because per-register analysis may allow to
> simplify the final code by reusing more already emitted vectors and
> better shuffles.
>
> Differential Revision: https://reviews.llvm.org/D148855

This reverts commit 9dfdbd78.