AVX-512: Optimized SIMD truncate operations for AVX512F set.

Optimized <8 x i32> to <8 x i16>
<4 x i64> to < 4 x i32>
<16 x i16> to <16 x i8>
All these oprtrations use now AVX512F set (KNL). Before this change it was implemented with AVX2 set.


Differential Revision: http://reviews.llvm.org/D14108

llvm-svn: 251764