Auto merge of #142839 - oli-obk:denullarification, r=RalfJung,celinval

Stop backends from needing to support nullary intrinsics

And then remove our infrastructure special casing them. Further improvements can now be done to them by avoiding the intermediate ConstValue step, but let's leave that to follow up work

r? `@RalfJung`

Author: bors

tests/pass/float.rs

@@ -1358,7 +1358,7 @@ fn test_min_max_nondet() {
     /// Ensure that if we call the closure often enough, we see both `true` and `false.`
     #[track_caller]
     fn ensure_both(f: impl Fn() -> bool) {
-        let rounds = 16;
+        let rounds = 32;
         let first = f();
         for _ in 1..rounds {
             if f() != first {