[llvm] [UpdateTestChecks] Don't fail silently when conflicting CHECK lines means no checks are generated for some functions (PR #159321)

[Alexander Richardson via llvm-commits]

================
@@ -917,15 +918,49 @@ def __init__(self, run_list, flags, scrubber_args, path, ginfo):
                 self._func_order.update({prefix: []})
                 self._global_var_dict.update({prefix: dict()})
 
+    # Return true if there is conflicting output for different runs for the
+    # given prefix and function name.
+    def has_conflicting_output(self, prefix, func):
+        # There was conflicting output if the func_dict is None for this
+        # prefix and function.
+        return self._func_dict[prefix].get(func) is None
+
     def finish_and_get_func_dict(self):
-        for prefix in self.get_failed_prefixes():
-            warn(
-                "Prefix %s had conflicting output from different RUN lines for all functions in test %s"
-                % (
-                    prefix,
-                    self._path,
+        all_funcs = set()
+        for prefix in self._func_dict:
+            all_funcs.update(self._func_dict[prefix].keys())
+
+        warnings_to_print = collections.defaultdict(list)
+        for func in sorted(list(all_funcs)):
+            for i, run_info in enumerate(self._run_list):
+                prefixes = run_info[0]
+                if not prefixes:
+                    continue
+
+                # Check if this RUN line produces this function at all. If
+                # not, we can skip analysing this function for this RUN.
+                run_contains_func = all(func in self._func_dict.get(p, {}) for p in prefixes)
+                if not run_contains_func:
+                    continue
+
+                # Check if this RUN line can print any checks for this
+                # function. It can't if all of its prefixes have conflicting
+                # (None) output.
+                can_print_for_this_run = not all(self.has_conflicting_output(p, func) for p in prefixes)
+                if not can_print_for_this_run:
----------------
arichardson wrote:

```suggestion
                cannot_print_for_this_run = all(self.has_conflicting_output(p, func) for p in prefixes)
                if cannot_print_for_this_run:
```
Maybe this to avoid the double not?

https://github.com/llvm/llvm-project/pull/159321