[llvm] [Float2Int] Resolve FIXME: Pick the smallest legal type that fits (PR #79158)

[via llvm-commits]

AtariDreams wrote:

> > I was able to reproduce the difference before and after the revert of the return value being different
> > edit: on x86-64 with `clang -O3`
> 
> Oddly enough, the pass itself doesn't seem to be the issue.

The -passes=float2int one I mean

https://github.com/llvm/llvm-project/pull/79158