Rollup merge of #125038 - ivan-shrimp:checked_sub, r=joboet

Invert comparison in `uN::checked_sub`

After #124114, LLVM no longer combines the comparison and subtraction in `uN::checked_sub` when either operand is a constant (demo: https://rust.godbolt.org/z/MaeoYbsP1). The difference is more pronounced when the expression is slightly more complex (https://rust.godbolt.org/z/4rPavsYdc).

This is due to the use of `>=` here:

ee97564e3a/library/core/src/num/uint_macros.rs (L581-L593)

For constant `C`, LLVM eagerly converts `a >= C` into `a > C - 1`, but the backend can only combine `a < C` with `a - C`, not `C - 1 < a` and `a - C`: e586556e37/llvm/lib/CodeGen/CodeGenPrepare.cpp (L1697-L1742)

This PR[^1] simply inverts the `>=` into `<` to restore the LLVM magic, and somewhat align this with the implementation of `uN::overflowing_sub` from #103299.

When the result is stored as an `Option` (rather than being branched/cmoved on), the discriminant is `self >= rhs`. This PR doesn't affect the codegen (and relevant tests) of that since LLVM will negate `self < rhs` to `self >= rhs` when necessary.

[^1]: Note to `self`: My very first contribution to publicly-used code. Hopefully like what I should learn to always be, tiny and humble.

library/core/src/num/uint_macros.rs

@@ -584,11 +584,11 @@ macro_rules! uint_impl {
             // Thus, rather than using `overflowing_sub` that produces a wrapping
             // subtraction, check it ourself so we can use an unchecked one.
 
-            if self >= rhs {
+            if self < rhs {
+                None
+            } else {
                 // SAFETY: just checked this can't overflow
                 Some(unsafe { intrinsics::unchecked_sub(self, rhs) })
-            } else {
-                None
             }
         }