[PATCH] [InstCombine] mark ADD with nuw if no unsigned overflow

[Mark Heffernan]

LGTM

================
Comment at: lib/Transforms/InstCombine/InstCombineAddSub.cpp:972
@@ +971,3 @@
+  // There are different heuristics we can use for this. Here are some simple
+  // ones.
+
----------------
nit: The comment suggests more than one technique for ruling out unsigned overflow, but only one is actually used.

http://reviews.llvm.org/D4144