Add loop pragmas

[vchuravy]

based on #31347, the loopinfo Expr allows us to communicate various pieces of information about loops to LLVM, in the C/OpenMP world these are know as loop pragmas.
They are optional metadata and LLVM can choose to discard it so it is not the best mechanism.

Mostly putting this up so that we can discuss the user-facing interface (which I have no strong feelings about.)

[DilumAluthge]

@vchuravy Can you rebase this on the latest master?

[DilumAluthge]

It looks like this PR has tests now, so I'm removing the "needs tests" label.

[vchuravy]

I think the big picture question here is what the user interface is supposed to be. I don't think we should have indivdual macros for each variant, and how does one note multiple loop pragmas at once?

[vchuravy]

Also documentation can be found here https://llvm.org/docs/TransformMetadata.html and here https://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations

[DilumAluthge]

Personally I don't mind the "individual macros" approach. But we could have a single macro @loopinfo as such:

Individual macro	Single macro
@unroll for	@loopinfo unroll for
@unroll 3 for	@loopinfo unroll=3 for
@jam for	@loopinfo jam for
@jam 5 for	@loopinfo jam=5 for

[DilumAluthge]

Btw, the LLVM docs don't quite match what we have in this PR? E.g. in this PR, @jam 3 for will set llvm.loop.unroll_and_jam.count to 3.

But https://llvm.org/docs/TransformMetadata.html does not mention llvm.loop.unroll_and_jam.count?

Presumably it sets the unroll factor. But it's not in the docs.

[vchuravy]

It's not mentioned there, but is the IR version of @pargma unroll_and_jam(4). With LLVM the rule is always trust the source ;)

[StefanKarpinski]

Needs API design work to make progress.

[carstenbauer]

FWIW, I would really like to see this merged. Anything I can help with?

[vchuravy]

I am really not happy with the UX design, but I haven't had any better ideas. You can see that I only exposed a few and then got frustrated with the plethora of names. There is also a compositionality question. E.g. @unroll @jam what does that mean can we support. So maybe we need to do something more like a @pragma :unroll...