AVX-512: Added all AVX-512 forms of Vector Convert for Float/Double/Int/Long types.

In this patch I have only encoding. Intrinsics and DAG lowering will be in the next patch.
I temporary removed the old intrinsics test (just to split this patch).
Half types are not covered here.

Differential Revision: http://reviews.llvm.org/D11134

llvm-svn: 242023