AVX-512: Floating point conversions for SKX - DAG Lowering.

SKX supports conversion for all FP types. Integer types include doublewords and quardwords.
I added "Legal" status for these nodes and a bunch of tests.
I added "NoVLX" for AVX DAG selection to force VLX instructions selection when VLX is supported.

Differential Revision: http://reviews.llvm.org/D11255

llvm-svn: 242637