Auto merge of #128250 - Amanieu:select_unpredictable, r=nikic

Add `select_unpredictable` to force LLVM to use CMOV

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

@@ -426,6 +426,7 @@ 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,