[SystemZ::TTI] Accurate costs for i1->double vector conversions

This factors out a new method getBoolVecToIntConversionCost() containing the
code for vector sext/zext of i1, in order to reuse it for i1 to double vector
conversions.

Review: Ulrich Weigand
https://reviews.llvm.org/D53923

llvm-svn: 345817