[llvm] [IR][Float8] Add two kinds float8 IR type (PR #89900)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > You're going to need a new RFC for this. Please check https://discourse.llvm.org/t/add-llvm-type-support-for-fp8-data-types-f8e4m3-and-f8e5m2/67598 and https://reviews.llvm.org/D140088 to get some background on why this has been rejected in this past.
> 
> so, we look to accept more stable data types rather than the unstable data types that come from machine learning? @nikic

The support surface area for something like this is really quite large. There's a lot of legalization work to support fundamental operations, which is kind of pointless if there is no underlying support on any target for them. Would first class, generic intrinsics for converting between each of these formats to other float types cover the real use cases? 

https://github.com/llvm/llvm-project/pull/89900