[llvm] [RFC][SPIR-V] Add llvm.arbitrary.fp.convert intrinsic (PR #164252)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

> Can you do int-to-int conversions

This is what is not intendent to be introduced, as well as "standard" float to "standard" float conversions. Originally what I was thinking of is something like what @arsenm suggested in https://github.com/llvm/llvm-project/pull/164252#discussion_r2445048292 , but then I came into question: how to represent mini-float to mini-float conversion. May be it's not that odd to have mini-floats as both input and output of [to/from] conversions intrinsics. 

https://github.com/llvm/llvm-project/pull/164252