Add a new compare_bytes intrinsic instead of calling memcmp directly

compiler/rustc_codegen_llvm/src/context.rs

@@ -891,7 +891,8 @@ impl<'ll> CodegenCx<'ll, '_> {
         ifn!("llvm.prefetch", fn(ptr, t_i32, t_i32, t_i32) -> void);
 
         // This isn't an "LLVM intrinsic", but LLVM's optimization passes
-        // recognize it like one and we assume it exists in `core::slice::cmp`
+        // recognize it like one (including turning it into `bcmp` sometimes)
+        // and we use it to implement intrinsics like `raw_eq` and `compare_bytes`
         match self.sess().target.arch.as_ref() {
             "avr" | "msp430" => ifn!("memcmp", fn(ptr, ptr, t_isize) -> t_i16),
             _ => ifn!("memcmp", fn(ptr, ptr, t_isize) -> t_i32),