Auto merge of rust-lang#125129 - cuviper:branchless-len_utf8, r=<try>

Remove the branches from `len_utf8`

This changes `len_utf8` to add all of the range comparisons together,
rather than branching on each one. We should definitely test performance
though, because it's possible that this will pessimize mostly-ascii
inputs that would have had a short branch-predicted path before.

Author: bors

library/core/src/char/methods.rs

@@ -1741,12 +1741,8 @@ impl EscapeDebugExtArgs {
 const fn len_utf8(code: u32) -> usize {
     if code < MAX_ONE_B {
         1
-    } else if code < MAX_TWO_B {
-        2
-    } else if code < MAX_THREE_B {
-        3
     } else {
-        4
+        2 + ((code >= MAX_TWO_B) as usize) + ((code >= MAX_THREE_B) as usize)
     }
 }