Revert "[SLP] avoid reduction transform on patterns that the backend can load-combine"

Revert "[SLP] avoid reduction transform on patterns that the backend can load-combine"

This reverts SVN r373833, as it caused a failed assert "Non-zero loop
cost expected" on building numerous projects, see PR43582 for details
and reproduction samples.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373882 91177308-0d34-0410-b5e6-96231b3b80d8