[Utils] Improved chunk-print-before-all.py script

[dbakunevich]

Summary

As part of this patch, the ability to work with dump files
containing ir after passes received using the -print-after-all
option was added to script chunk-print-before-all.py.
The names of the output files have also been changed.

Before: chunk-NUM.ll
Now: NUM-STAGE-PASS_NAME.ll

NUM - sequence number of the pass operation.
STAGE - stage of the pass operation (for example: before, after, dump).
PASS_NAME - the name of the pass.

[apilipenko]

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

1–2

Update the comment

43–48

Extract into a helper and add a comment on what this code is doing?

[apilipenko]

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

15–19

This should do the trick:

return re.split(' |<', short_line)[0]