[llvm] r243779 - IR: Add a broad bitcode compatibility test

[Vedant Kumar]

I've responded to comments inline -- two patches attached.

>> 
>> -* The bitcode format produced by a X.Y release will be readable by all following
>> -  X.Z releases and the (X+1).0 release.
>> +* Additions and changes to the IR should be reflected in
>> +  ``test/Bitcode/compatibility.ll``.
>> +
>> +* The bitcode format produced by a X.Y release will be readable by all
>> +  following X.Z releases and the (X+1).0 release. To help ensure this, an X.Y
>> +  version of ``test/Bitcode/compatibility.ll`` should be assembled and
>> +  committed after each release.
>> 
>> Please be a bit more specific here. I would like us to document explicitly the naming of the assembled files to avoid guesswork in the future.

Here's the new language:

> 508 * Additions and changes to the IR should be reflected in
> 509   ``test/Bitcode/compatibility.ll``.
> 510 
> 511 * The bitcode format produced by a X.Y release will be readable by all
> 512   following X.Z releases and the (X+1).0 release.
> 513 
> 514 * After each X.Y release, ``compatibility.ll`` must be copied to
> 515   ``compatibility-X.Y.ll``. The corresponding bitcode file should be assembled
> 516   using the X.Y build and committed as ``compatibility-X.Y.ll.bc``.



>> + at comdat.samesize = global i32 0, comdat
>> +; CHECK: @comdat.samesize = global i32 0, comdat
>> 
>> Is there a reason this is not covering usage of comdats with explicit comdat name?

Not a good one. At one point I had mistakenly assumed that 3.6 lacked support for this.

``compatibility.ll`` has coverage for this, so I've copied it into the 3.6 test.

[1] fix-combat-coverage.patch

[Bitcode] Cover explicit comdat names in compatibility-3.6.ll

[2] fix-devpolicy-language.patch

[docs] Language about managing compatibility.ll made clearer

 
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