Revert "[SLP]Improve costs in computeExtractCost() to avoid crash after D158449."

This caused asserts:

  Assertion failed: NumElts > 1 && "Expected at least 2-element fixed length vector(s).",
  file C:\b\s\w\ir\cache\builder\src\third_party\llvm\llvm\lib\Transforms\Vectorize\SLPVectorizer.cpp, line 7096

see comment on https://github.com/llvm/llvm-project/commit/59a67ea35d608480257fc64ec3e5106ef50de740

> Need to consider the length of the original vector for extractelements,
> not the length, matched number of the scalars. It fixes 2 issues: 1)
> improves cost estimation; 2) Fixes crashes after D158449.

This reverts commit 59a67ea3.