Add Saturation Arithmetic Intrinsics

[baziotis]

Solving issue: ldc-developers/ldc#2925

[kinke]

Thx. These intrinsics have apparently been added with LLVM 8 (not present in http://releases.llvm.org/7.0.0/docs/LangRef.html, but in http://releases.llvm.org/8.0.0/docs/LangRef.html), so please guard with version (INTRINSICS_FROM_800).

[baziotis]

Done, I don't know how you prefer the indentation though.

[thewilsonator]

D files in the D style, C++ in the LLVM style.

[baziotis]

Yes, but I've seen version in different flavors. From what I see, I think it's ok (i.e. the other version (INTRINSICS...) follow the same indentation.