[PATCH] add a convenience method to copy wrapping, exact, and fast-math flags (NFC)

[Arnold Schwaighofer]

LGTM.

Thanks!

================
Comment at: lib/IR/Instructions.cpp:2038
@@ +2037,3 @@
+    setHasNoUnsignedWrap(OB->hasNoUnsignedWrap());
+  }
+  // Copy the exact flag.
----------------
Nitpick: Can you put a newline below this line for symmetry?

http://reviews.llvm.org/D5138