Includes module header first.

This "include module header first" is part of the llvm style guide.

This change exposes "broken" headers that were accessing undefined types, or
even using types without forward declaring them.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1202253002.
diff --git a/src/IceOperand.cpp b/src/IceOperand.cpp
index db530db..60a73d9 100644
--- a/src/IceOperand.cpp
+++ b/src/IceOperand.cpp
@@ -12,10 +12,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "IceOperand.h"
+
 #include "IceCfg.h"
 #include "IceCfgNode.h"
 #include "IceInst.h"
-#include "IceOperand.h"
 #include "IceTargetLowering.h" // dumping stack/frame pointer register
 
 namespace Ice {