[TTI] Reduction costs only need to include a single extract element cost (REAPPLIED)

We were adding the entire scalarization extraction cost for reductions, which returns the total cost of extracting every element of a vector type.

For reductions we don't need to do this - we just need to extract the 0'th element after the reduction pattern has completed.

Fixes PR37731

Rebased and reapplied after being reverted in rL347541 due to PR39774 - which was fixed by D54955/rL347759 and D55017/rL347997

Differential Revision: https://reviews.llvm.org/D54585

llvm-svn: 348076