[llvm-dev] [RFC] Add DebugLoc parameter in Instruction’s Create() functions

[Anast Gramm via llvm-dev]

Many DI-related bugs are caused by missing Debug Location
in an instruction created in a transformation. Most of the
time the fix is trivial once you found where the culprit
instruction is created (https://reviews.llvm.org/D50263).
Currently, when you create a new Instruction, in order to
give it DL you have to either use an IRBuilder that is
previously set to the correct DL or “manually” create
the instruction via one of it’s Create() routines and then
call `setDebugLoc()` to it.

I propose the addition of a DebugLoc parameter
in the *::Create() instruction constructors.
This could be in the form of a pure DebugLoc
variable or possibly an `Instruction *InheritLocationFromInst` one

Some pros of this idea are:
 - Easier to create instructions with debug location.
 - It will make the code more readable by eliminating
 the many `NewInst->setDebugLoc(OldInst->getDebugLoc)` calls.
 - It will incentivize people to think about the DebugLoc of
 their newly created instruction and where it should come from.

As the Create() functions are widely used, and sometimes
location data could be out of scope when the Instruction
is created, I think the best approach to this is to
introduce the new field as optional that defaults to
empty DL (as it is now) to avoid huge refactoring,
and to allow more time for testing individual changes.
Refactoring could then be done in steps and the
parameter could become mandatory in some ::Create()
constructors as we judge fit.

With that in mind here are some cons:
 - Incomplete transition of APIs.
 - Lack of infrastructure to check that the locations
 supplied to *::Create are correct.

What do you think?