[X86][AVX512] Replace lossless i32/u32 to f64 conversion intrinsics with generic IR

Both the (V)CVTDQ2PD (i32 to f64) and (V)CVTUDQ2PD (u32 to f64) conversion instructions are lossless and can be safely represented as generic __builtin_convertvector calls instead of x86 intrinsics without affecting final codegen.

This patch removes the clang builtins and their use in the headers - a future patch will deal with removing the llvm intrinsics.

This is an extension patch to D20528 which dealt with the equivalent sse/avx cases.

Differential Revision: https://reviews.llvm.org/D26686

llvm-svn: 287088