[Transforms][Debugify] Fix "Missing line" false alarm on PHI nodes

This is a fix for https://bugs.llvm.org/show_bug.cgi?id=49959

The "Missing line" false alarm was introduced in D75242.

Patch by Yilong Guo<yilong.guo@intel.com>

Differential Revision: https://reviews.llvm.org/D100446