Revert using regex Pattern in String.replace

Use String.indexOf(..., ignoreCase) instead in all branches to preserve
compatibility with behavior before 1.4.20 that used String.split which
essentially relied on that String.indexOf

#KT-43745 Fixed

libraries/stdlib/test/text/StringTest.kt

@@ -896,6 +896,22 @@ class StringTest {
 
         assertEquals("-a-b-b-A-b-", input.replace("", "-"))
         assertEquals("-a-b-b-A-b-", input.replace("", "-", ignoreCase = true))
+
+        fun testIgnoreCase(chars: String) {
+            for ((i, c) in chars.withIndex()) {
+                val message = "Char: $c (${c.toInt()})"
+                val expectOneReplaced = chars.replaceRange(i..i, "_")
+                val expectAllReplaced = "_".repeat(chars.length)
+                assertEquals(expectOneReplaced, chars.replace(c, '_'), message)
+                assertEquals(expectAllReplaced, chars.replace(c, '_', ignoreCase = true), "$message, ignoreCase")
+                assertEquals(expectOneReplaced, chars.replace(c.toString(), "_"), "$message, as string")
+                assertEquals(expectAllReplaced, chars.replace(c.toString(), "_", ignoreCase = true), "$message, as string, ignoreCase")
+            }
+        }
+
+        testIgnoreCase("üÜ")
+        testIgnoreCase("öÖ")
+        testIgnoreCase("äÄ")
     }
 
     @Test fun replaceFirst() {