[Debugify][Original DI] Test dbg var loc preservation

This is an improvement of [0]. This adds checking of
original llvm.dbg.values()/declares() instructions in
optimizations.

We have picked a real issue that has been found with
this (actually, picked one variable location missing
from [1] and resolved the issue), and the result is
the fix for that -- D100844.

Before applying the D100844, using the options from [0]
(but with this patch applied) on the compilation of GDB 7.11,
the final HTML report for the debug-info issues can be found
at [1] (please scroll down, and look for
"Summary of Variable Location Bugs"). After applying
the D100844, the numbers has improved a bit -- please take
a look into [2].

[0] https://llvm.org/docs/HowToUpdateDebugInfo.html\
[1] https://djolertrk.github.io/di-check-before-adce-fix/
[2] https://djolertrk.github.io/di-check-after-adce-fix/

Differential Revision: https://reviews.llvm.org/D100845