Auto merge of #116281 - Nadrieril:eager-const-eval, r=cjgillot

Cleanup number handling in match exhaustiveness

Doing a little bit of cleanup; handling number constants was somewhat messy. In particular, this:

- evals float consts once instead of repetitively
- reduces `Constructor` from 88 bytes to 56 (`mir::Const` is big!)

The `fast_try_eval_bits` function was mostly constructed from inlining existing code but I don't fully understand it; I don't follow how consts work and are evaluated very well.

compiler/rustc_middle/src/mir/consts.rs

@@ -288,7 +288,16 @@ impl<'tcx> Const<'tcx> {
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Option<ScalarInt> {
-        self.try_eval_scalar(tcx, param_env)?.try_to_int().ok()
+        match self {
+            // If the constant is already evaluated, we shortcut here.
+            Const::Ty(c) if let ty::ConstKind::Value(valtree) = c.kind() => {
+                valtree.try_to_scalar_int()
+            },
+            // This is a more general form of the previous case.
+            _ => {
+                self.try_eval_scalar(tcx, param_env)?.try_to_int().ok()
+            },
+        }
     }
 
     #[inline]