[AVR][NFC] Supplement a test for inline assembly constraint 'I'

Author: benshi001

clang/lib/Basic/Targets/AVR.h

@@ -124,6 +124,8 @@ class LLVM_LIBRARY_VISIBILITY AVRTargetInfo : public TargetInfo {
       Info.setAllowsRegister();
       return true;
     case 'I': // 6-bit positive integer constant
+      // Due to issue https://github.com/llvm/llvm-project/issues/51513, we
+      // allow value 64 in the frontend and let it be dinied in the backend.
       Info.setRequiresImmediate(0, 64);
       return true;
     case 'J': // 6-bit negative integer constant

llvm/test/CodeGen/AVR/inline-asm/inline-asm-invalid.ll

@@ -21,3 +21,9 @@ define void @foo2() {
   call void asm sideeffect "ldd r24, X+2", ""()
   ret void
 }
+
+define void @foo3() {
+  ; AVR6: error: value out of range for constraint 'I'
+  call void asm sideeffect "out $0, r20", "I"(i16 64)
+  ret void
+}