document some subtlety

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33257 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86Subtarget.h

@@ -32,7 +32,9 @@ enum Style {
 class X86Subtarget : public TargetSubtarget {
 public:
   enum AsmWriterFlavorTy {
-    ATT, Intel, Unset
+    // Note: This numbering has to match the GCC assembler dialects for inline
+    // asm alternatives to work right.
+    ATT = 0, Intel = 1, Unset
   };
 protected:
   enum X86SSEEnum {