Auto merge of #116551 - RalfJung:nondet-nan, r=oli-obk

miri: make NaN generation non-deterministic

This implements the [LLVM semantics for NaN generation](https://llvm.org/docs/LangRef.html#behavior-of-floating-point-nan-values). I will soon submit an RFC to make this also officially the Rust semantics, but it has been our de-facto semantics for a long time so there's no reason Miri has to wait for that RFC. This PR just better aligns Miri with codegen.

This PR does that just for the operations that have MIR primitives; a future PR will adjust the intrinsics.

Author: bors