[LV] Use getScalarizationOverhead in memory instruction costs (NFC)

[LV] Use getScalarizationOverhead in memory instruction costs (NFC)

This patch refactors the cost estimation of scalarized loads and stores to
reuse getScalarizationOverhead for the cost of the extractelement and
insertelement instructions we might create. The existing code accounted for
this cost, but it was functionally equivalent to the helper function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283364 91177308-0d34-0410-b5e6-96231b3b80d8