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/builder.rs

@@ -1353,6 +1353,16 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
         }
     }
 
+    pub fn set_unpredictable(&mut self, inst: &'ll Value) {
+        unsafe {
+            llvm::LLVMSetMetadata(
+                inst,
+                llvm::MD_unpredictable as c_uint,
+                llvm::LLVMMDNodeInContext(self.cx.llcx, ptr::null(), 0),
+            );
+        }
+    }
+
     pub fn minnum(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
         unsafe { llvm::LLVMRustBuildMinNum(self.llbuilder, lhs, rhs) }
     }