[PATCH] Intrin: Add _umul128

[Hans Wennborg]

lgtm with comment

================
Comment at: lib/Headers/Intrin.h:419
@@ -418,2 +418,3 @@
 unsigned __int64 _shlx_u64(unsigned __int64, unsigned int);
 unsigned __int64 shrx_u64(unsigned __int64, unsigned int);
+static __inline__ unsigned __int64 __attribute__((__always_inline__, __nodebug__))
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While you're here, is shrx_u64 missing an underscore?

================
Comment at: lib/Headers/Intrin.h:423
@@ +422,3 @@
+         unsigned __int64 *_HighProduct) {
+  __int128 FullProduct =
+      (unsigned __int128)_Multiplier * (unsigned __int128)_Multiplicand;
----------------
use an underscore for FullProduct to be safe?

http://reviews.llvm.org/D6486