Force LLVM to use CMOV for binary search

Since https://reviews.llvm.org/D118118, LLVM will no longer turn CMOVs
into branches if it comes from a `select` marked with an `unpredictable`
metadata attribute.

This PR introduces `core::intrinsics::select_unpredictable` which emits
such a `select` and uses it in the implementation of `binary_search_by`.

compiler/rustc_span/src/symbol.rs

@@ -1701,6 +1701,7 @@ symbols! {
         saturating_add,
         saturating_div,
         saturating_sub,
+        select_unpredictable,
         self_in_typedefs,
         self_struct_ctor,
         semitransparent,