Avoid superfluous UB checks in IndexRange

`IndexRange::len` is justified as an overall invariant, and
`take_prefix` and `take_suffix` are justified by local branch
conditions. A few more UB-checked calls remain in cases that are only
supported locally by `debug_assert!`, which won't do anything in
distributed builds, so those UB checks may still be useful.

We generally expect core's `#![rustc_preserve_ub_checks]` to optimize
away in user's release builds, but the mere presence of that extra code
can sometimes inhibit optimization, as seen in rust-lang#131563.

Author: cuviper

core/src/ops/index_range.rs

@@ -45,7 +45,8 @@ impl IndexRange {
     #[inline]
     pub const fn len(&self) -> usize {
         // SAFETY: By invariant, this cannot wrap
-        unsafe { self.end.unchecked_sub(self.start) }
+        // Using the intrinsic because a UB check here impedes LLVM optimization. (#131563)
+        unsafe { crate::intrinsics::unchecked_sub(self.end, self.start) }
     }
 
     /// # Safety
@@ -82,7 +83,8 @@ impl IndexRange {
         let mid = if n <= self.len() {
             // SAFETY: We just checked that this will be between start and end,
             // and thus the addition cannot overflow.
-            unsafe { self.start.unchecked_add(n) }
+            // Using the intrinsic avoids a superfluous UB check.
+            unsafe { crate::intrinsics::unchecked_add(self.start, n) }
         } else {
             self.end
         };
@@ -100,8 +102,9 @@ impl IndexRange {
     pub fn take_suffix(&mut self, n: usize) -> Self {
         let mid = if n <= self.len() {
             // SAFETY: We just checked that this will be between start and end,
-            // and thus the addition cannot overflow.
-            unsafe { self.end.unchecked_sub(n) }
+            // and thus the subtraction cannot overflow.
+            // Using the intrinsic avoids a superfluous UB check.
+            unsafe { crate::intrinsics::unchecked_sub(self.end, n) }
         } else {
             self.start
         };