[AVX512] Add intrinsics for FP scalar broadcasts

Similar approach to the set1 intrinsics is used: implement in terms of vector
initializers and then ensure with an LLVM test that a broadcast is generated
at the end.

Part of <rdar://problem/17688758>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215486 91177308-0d34-0410-b5e6-96231b3b80d8