LLVM calling convention for AVX2 and AVX512 registers

[Edward Kmett]

Back around 2013, Geoff raised a discussion about fixing up the GHC ABI so
that the LLVM calling convention could pass 256 bit vector types in YMM
(and, i suppose now 512 bit vector types in ZMM).

As I recall, this was blocked by some short term concerns about which LLVM
release was imminent or what have you. Four years on, the exact same sort
of arguments could be dredged up, but yet in the meantime nobody is really
using those types for anything.

This still creates a pain point around trying to use these wide types
today. Spilling rather than passing them in registers adds a LOT of
overhead to any attempt to use them that virtually erases any benefit to
having them in the first place.

I started experimenting with writing some custom primops directly in llvm
so I could do meaningful amounts of work with our SIMD vector types by just
banging out the code that we can't write in haskell directly using llvm
assembly, and hoping I could trick LLVM to do link time optimization to
perhaps inline it, but I'm basically dead in the water over the overhead of
our current calling convention, before I even start, it seems, as if we're
spilling them there is no way that inlining / LTO could hope to figure out
what we're doing out as part of the spill to erase that call entirely.

It is rather frustrating that I can't even cheat. =/

What do we need to do to finally fix this?

-Edward
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