[SystemZ] Rework getInterleavedMemoryOpCost()

Model this function more closely after the BasicTTIImpl version, with
separate handling of loads and stores. For loads, the set of actually loaded
vectors is checked.

This makes it more readable and just slightly more accurate generally.

Review: Ulrich Weigand
https://reviews.llvm.org/D53071

llvm-svn: 345998