diff --git a/llvm/utils/update_mir_test_checks.py b/llvm/utils/update_mir_test_checks.py
--- a/llvm/utils/update_mir_test_checks.py
+++ b/llvm/utils/update_mir_test_checks.py
@@ -96,7 +96,7 @@
     return None
 
 
-def build_run_list(test, run_lines, verbose=False):
+def build_run_list(test, run_lines, args):
     run_list = []
     all_prefixes = []
     for l in run_lines:
@@ -142,8 +142,23 @@
     # Remove any common prefixes. We'll just leave those entirely alone.
     common_prefixes = set([prefix for prefix in all_prefixes
                            if all_prefixes.count(prefix) > 1])
-    for run in run_list:
-        run.prefixes = [p for p in run.prefixes if p not in common_prefixes]
+
+    # ...unless we're told to use the first one we see, ignoring the rest.
+    used_prefixes = set()
+    if args.use_first_common_prefix:
+        for run in run_list:
+            filtered_prefixes = []
+            for p in run.prefixes:
+                if p in common_prefixes:
+                    if p not in used_prefixes:
+                        filtered_prefixes.append(p)
+                        used_prefixes.add(p)
+                else:
+                    filtered_prefixes.append(p)
+        run.prefixes = filtered_prefixes
+    else:
+        for run in run_list:
+            run.prefixes = [p for p in run.prefixes if p not in common_prefixes]
 
     return run_list, common_prefixes
 
@@ -297,7 +312,7 @@
 
     triple_in_ir = find_triple_in_ir(input_lines, args.verbose)
     run_lines = common.find_run_lines(test, input_lines)
-    run_list, common_prefixes = build_run_list(test, run_lines, args.verbose)
+    run_list, common_prefixes = build_run_list(test, run_lines, args)
 
     simple_functions = find_functions_with_one_bb(input_lines, args.verbose)
 
@@ -325,7 +340,7 @@
 
     if args.remove_common_prefixes:
         prefix_set.update(common_prefixes)
-    elif common_prefixes:
+    elif common_prefixes and not args.use_first_common_prefix:
         common.warn('Ignoring common prefixes: {}'.format(common_prefixes),
              test_file=test)
 
@@ -418,6 +433,9 @@
     parser.add_argument('--remove-common-prefixes', action='store_true',
                         help='Remove existing check lines whose prefixes are '
                              'shared between multiple commands')
+    parser.add_argument('--use-first-common-prefix', action='store_true',
+                        help='If there are common prefixes shared between run lines,'
+                        ' use the first one instead of ignoring them all')
     parser.add_argument('tests', nargs='+')
     args = common.parse_commandline_args(parser)