* std.math.snan: fix compilation error. Also make it and nan inline.
 * LLVM: use a proper enum type for float op instead of enum literal.
   Also various cleanups.
 * LLVM: use LLVMBuildVectorSplat for vector splat AIR instruction.
   - also the bindings had parameter order wrong
 * LLVM: additionally handle f16 lowering. For now all targets report OK
   but I think we will need to add some exceptions to this list.