[PATCH] Intrin: Add _umul128

[Reid Kleckner]

lgtm

Nice!

================
Comment at: lib/Headers/Intrin.h:423
@@ +422,3 @@
+         unsigned __int64 *_HighProduct) {
+  __int128 FullProduct =
+      (unsigned __int128)_Multiplier * (unsigned __int128)_Multiplicand;
----------------
hans wrote:
> use an underscore for FullProduct to be safe?
Why not unsigned __int128? I believe this affects the shift right below, but all the bits that would be 1 are truncated out.

http://reviews.llvm.org/D6486