[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