[utils] Fix regexp in llvm/utils/extract_vplan.py to extract VPlans.

Regarding this bug in Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=46451

I went ahead and fixed the regexp pattern and now Python script is able
to extract vplan graphs from the log files. Additionally some test for
this would be nice to have but I'm not sure are Python scripts tested
in LLVM and if so where they live.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D86068