Use minnum/maxnum LLVM instrinsics for fmin/fmax

[huonw]

http://reviews.llvm.org/rL220341

Requires an LLVM upgrade, but not worth doing one just for this purpose.

[huonw]

(Many LLVM upgrades have happened since then.)

[varkor]

Update as of last 2017: there are still issues with optimisation of minnum/maxnum when used for clamping on x86, which seems to be the only blocker. There's an implementation here that could be merged when LLVM figures out how to optimise that case.

[nox]

clamp01_llvm is still worse than clamp01_rust. Is there any upstream issue on LLVM that we could refer to here?

Cc @rust-lang/wg-codegen

[nikic]

The X86 codegen issue should be fixed by llvm/llvm-project@d87eced.