[Tools] Fixed bug with llvm/utils/chunk-print-before-all.py script.

Prior to the fix, the script was not annotating the first line of
chunk-0.ll. Because of that, a compilation with ./bin/opt was failing.

The extra if-statement ensures that the corner case is covered

Reviewed-By: apilipenko

Differential Revision: https://reviews.llvm.org/D76507

llvm/utils/chunk-print-before-all.py

@@ -24,9 +24,10 @@ def print_chunk(lines):
 is_dump = False
 cur = []
 for line in sys.stdin:
-    if line.startswith("*** IR Dump Before ") and len(cur) != 0:
-        print_chunk(cur);
-        cur = []
+    if line.startswith("*** IR Dump Before "):
+        if len(cur) != 0:
+            print_chunk(cur);
+            cur = []
         cur.append("; " + line)
     elif line.startswith("Stack dump:"):
         print_chunk(cur);