Add test for iterating over MDNode operands when they are empty (#80737)

With e851278 the for loop that iterates
over MDNode operands was changed to a range-based for loop. This change
surfaces a bug where if the result of MD->operands() is an ArrayRef that
has a size of 0, then iterating over that ArrayRef leads to a
segmentation fault, due to accessing invalid addresses.
    
This was reverted with 6ce03ff but this
test should be added to test that codepath in the future.

llvm/test/Verifier/verify-dwarf-no-operands.ll

@@ -0,0 +1,25 @@
+; RUN: llvm-as -disable-output %s
+%"class.llvm::StringRef" = type { ptr, i64 }
+define internal void @_ZL30tokenizeWindowsCommandLineImplN4llvm9StringRefERNS_11StringSaverENS_12function_refIFvS0_EEEbNS3_IFvvEEEb() !dbg !12 {
+  %7 = alloca %"class.llvm::StringRef", align 8
+  %21 = call noundef i64 @_ZNK4llvm9StringRef4sizeEv(ptr noundef nonnull align 8 dereferenceable(16) %7)
+  br label %22
+  br label %22, !llvm.loop !284 ; This instruction has loop metadata but no operands and should not result in a segmentation fault in the verifier.
+}
+define linkonce_odr noundef i64 @_ZNK4llvm9StringRef4sizeEv() align 2 !dbg !340 {
+  %2 = alloca ptr, align 8
+  %3 = load ptr, ptr %2, align 8
+  %4 = getelementptr inbounds %"class.llvm::StringRef", ptr %3
+  %5 = load i64, ptr %4
+  ret i64 %5
+}
+!llvm.module.flags = !{!2, !6}
+!llvm.dbg.cu = !{!7}
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = !{i32 7, !"frame-pointer", i32 1}
+!7 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !8, sdk: "MacOSX14.0.sdk")
+!8 = !DIFile(filename: "file.cpp", directory: "/Users/Dev", checksumkind: CSK_MD5, checksum: "ed7ae158f20f7914bc5fb843291e80da")
+!12 = distinct !DISubprogram(unit: !7, retainedNodes: !36)
+!36 = !{}
+!284 = distinct !{}
+!340 = distinct !DISubprogram(unit: !7, retainedNodes: !36)