[X86] Make some cast costs more precise

Make some AVX and AVX512 cast costs more precise.
Based on part of a patch by Elena Demikhovsky (D15604).

Differential Revision: http://reviews.llvm.org/D22064


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275106 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed