Auto merge of rust-lang#81354 - SkiFire13:binary-search-assume, r=nagisa

Instruct LLVM that binary_search returns a valid index

This allows removing bound checks when the return value of `binary_search` is used to index into the slice it was call on. I also added a codegen test for this, not sure if it's the right thing to do (I didn't find anything on the dev guide), but it felt so.

Author: bors

core/src/slice/mod.rs

@@ -2204,6 +2204,8 @@ impl<T> [T] {
             } else if cmp == Greater {
                 right = mid;
             } else {
+                // SAFETY: same as the `get_unchecked` above
+                unsafe { crate::intrinsics::assume(mid < self.len()) };
                 return Ok(mid);
             }