Optimize out no-op yield from statements

If the array is empty, then I'd expect that the generator is never left,
and that can be converted to a no-op and the return value would always be `null`.

Make `yield from [];` as efficient as `if (false) { yield null; }`
when opcache's sccp pass is enabled.

Closes GH-5679

ext/opcache/Optimizer/sccp.c

@@ -1678,6 +1678,16 @@ static void sccp_visit_instr(scdf_ctx *scdf, zend_op *opline, zend_ssa_op *ssa_o
 			}
 			SET_RESULT_BOT(result);
 			break;
+		case ZEND_YIELD_FROM:
+			// tmp = yield from [] -> tmp = null
+			SKIP_IF_TOP(op1);
+			if (Z_TYPE_P(op1) == IS_ARRAY && zend_hash_num_elements(Z_ARR_P(op1)) == 0) {
+				ZVAL_NULL(&zv);
+				SET_RESULT(result, &zv);
+				break;
+			}
+			SET_RESULT_BOT(result);
+			break;
 		case ZEND_COUNT:
 			SKIP_IF_TOP(op1);
 			if (Z_TYPE_P(op1) == IS_ARRAY) {

ext/opcache/tests/opt/sccp_032.phpt

@@ -0,0 +1,56 @@
+--TEST--
+SCCP 032: Yield from optimizations
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.optimization_level=-1
+opcache.opt_debug_level=0x20000
+opcache.preload=
+--SKIPIF--
+<?php require_once('skipif.inc'); ?>
+--FILE--
+<?php
+function test(): Generator {
+    $result = yield from [];
+    $a = [];
+    yield from $a;
+    yield $result;
+    $a[] = 3;
+    yield from $a;
+}
+foreach (test() as $x) {
+    var_export($x);
+    echo "\n";
+}
+?>
+--EXPECTF--
+$_main:
+     ; (lines=11, args=0, vars=1, tmps=2)
+     ; (after optimizer)
+     ; %ssccp_032.php:1-15
+0000 INIT_FCALL 0 %d string("test")
+0001 V2 = DO_UCALL
+0002 V1 = FE_RESET_R V2 0009
+0003 FE_FETCH_R V1 CV0($x) 0009
+0004 INIT_FCALL 1 %d string("var_export")
+0005 SEND_VAR CV0($x) 1
+0006 DO_ICALL
+0007 ECHO string("
+")
+0008 JMP 0003
+0009 FE_FREE V1
+0010 RETURN int(1)
+LIVE RANGES:
+     1: 0003 - 0009 (loop)
+
+test:
+     ; (lines=5, args=0, vars=0, tmps=1)
+     ; (after optimizer)
+     ; %ssccp_032.php:2-9
+0000 GENERATOR_CREATE
+0001 YIELD null
+0002 T0 = YIELD_FROM array(...)
+0003 FREE T0
+0004 GENERATOR_RETURN null
+NULL
+3