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

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

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

llvm-svn: 346970