[UTC] Recognise CHECK lines with globals matched literally (#70050)

Previously when using `-p` a.k.a. `--preserve-names` existing lines for
checking globals were not recognised as such, leading to the line being
kept while also being emitted again, resulting in duplicated CHECK
lines.

This resolves #70048.

Author: hnrklssn

llvm/test/tools/UpdateTestChecks/update_test_checks/Inputs/global_preserve_name.ll

@@ -0,0 +1,13 @@
+; RUN: opt -S < %s | FileCheck %s
+
+@G = constant i32 42
+
+;.
+; CHECK: @G = constant i32 42
+;.
+define ptr @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    ret ptr @G
+;
+  ret ptr @G
+}

llvm/test/tools/UpdateTestChecks/update_test_checks/Inputs/global_preserve_name.ll.expected

@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: -p --check-globals
+; RUN: opt -S < %s | FileCheck %s
+
+@G = constant i32 42
+
+;.
+; CHECK: @G = constant i32 42
+;.
+define ptr @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    ret ptr @G
+;
+  ret ptr @G
+}

llvm/test/tools/UpdateTestChecks/update_test_checks/global_preserve_name.test

@@ -0,0 +1,7 @@
+## Basic test checking that we capture existing lines matching global variable names
+# RUN: cp -f %S/Inputs/global_preserve_name.ll %t.ll && %update_test_checks %t.ll --check-globals --preserve-names
+# RUN: diff -u %t.ll %S/Inputs/global_preserve_name.ll.expected
+## Verify that running without the --global-value-regex flag respects UTC_ARGS, and that the output is a fixed point.
+# RUN: %update_test_checks %t.ll
+# RUN: diff -u %t.ll %S/Inputs/global_preserve_name.ll.expected
+

llvm/utils/UpdateTestChecks/common.py

@@ -391,7 +391,7 @@ def should_add_line_to_output(
     m = CHECK_RE.match(input_line)
     if m and m.group(1) in prefix_set:
         if skip_global_checks:
-            global_ir_value_re = re.compile(r"\[\[", flags=(re.M))
+            global_ir_value_re = re.compile(r"(\[\[|@)", flags=(re.M))
             return not global_ir_value_re.search(input_line)
         return False