AVX-512: Updated cost of FP/SINT/UINT conversion operations

I checked and updated the cost of AVX-512 conversion operations. Added cost of conversion operations in DQ mode.
Conversion of illegal types that requires vector split is not calculated right now (like for other X86 targets).

Differential Revision: http://reviews.llvm.org/D15074

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254494 91177308-0d34-0410-b5e6-96231b3b80d8