[SystemZ::TTI] Improve accuracy of costs for vector fp <-> int conversions

Improve getCastInstrCost() by respecting the different types of Src and Dst
for vector integer <-> fp conversions.

This means that extracting from integer becomes more expensive (by the
extraction penalty), and the extraction from fp becomes cheaper (no longer
has a false extraction penalty).

Review: Ulrich Weigand
https://reviews.llvm.org/D54423

llvm-svn: 346663