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_codegen_llvm/src/llvm/ffi.rs

@@ -428,6 +428,7 @@ pub enum MetadataType {
     MD_nontemporal = 9,
     MD_mem_parallel_loop_access = 10,
     MD_nonnull = 11,
+    MD_unpredictable = 15,
     MD_align = 17,
     MD_type = 19,
     MD_vcall_visibility = 28,