ext/standard: Return the input of htmlspecialchars() when nothing needs encoding

htmlspecialchars() allocated a buffer of twice the input and decoded it
character by character, even when the result was the input itself.

The input is now scanned first for the characters that need encoding and
for invalid multi-byte sequences, with SSE2 on 16-byte blocks. When there
is none, the input string is returned, as html_entity_decode() and
htmlspecialchars_decode() already do since 68dc754. Otherwise, encoding
starts after the part that was scanned.

The @refcount 1 annotations of htmlspecialchars() and htmlentities() are
removed since they can now return their argument.

Author: nicolas-grekas

NEWS

@@ -21,5 +21,7 @@ PHP                                                                        NEWS
 - Standard:
   . Improved performance of array_splice() when inserting without removing
     elements. (mehmetcansahin)
+  . Improved performance of htmlspecialchars() when nothing needs encoding.
+    (Nicolas Grekas)
 
 <<< NOTE: Insert NEWS from last stable release here prior to actual release! >>>

UPGRADING

@@ -74,3 +74,5 @@ PHP 8.7 UPGRADE NOTES
 - Standard:
   . Improved performance of array_splice() when inserting without removing
     elements.
+  . htmlspecialchars() returns its input without copying it when nothing needs
+    encoding.

Zend/Optimizer/zend_func_infos.h

@@ -463,8 +463,6 @@ static const func_info_t func_infos[] = {
 	F1("inet_pton", MAY_BE_STRING|MAY_BE_FALSE),
 	F1("metaphone", MAY_BE_STRING),
 	F1("headers_list", MAY_BE_ARRAY|MAY_BE_ARRAY_KEY_LONG|MAY_BE_ARRAY_OF_STRING),
-	F1("htmlspecialchars", MAY_BE_STRING),
-	F1("htmlentities", MAY_BE_STRING),
 	F1("get_html_translation_table", MAY_BE_ARRAY|MAY_BE_ARRAY_KEY_STRING|MAY_BE_ARRAY_OF_STRING),
 	F1("bin2hex", MAY_BE_STRING),
 	F1("hex2bin", MAY_BE_STRING|MAY_BE_FALSE),

ext/standard/basic_functions.stub.php

@@ -2267,14 +2267,12 @@ function headers_list(): array {}
 
 /* {{{ html.c */
 
-/** @refcount 1 */
 function htmlspecialchars(string $string, int $flags = ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, ?string $encoding = null, bool $double_encode = true): string {}
 
 function htmlspecialchars_decode(string $string, int $flags = ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401): string {}
 
 function html_entity_decode(string $string, int $flags = ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, ?string $encoding = null): string {}
 
-/** @refcount 1 */
 function htmlentities(string $string, int $flags = ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML401, ?string $encoding = null, bool $double_encode = true): string {}
 
 /**