[JVM IR] Use append(Char) for 1-length string literals in string

templates and plus concatenations.

This is slightly more efficient and mirrors the behavior of the non-IR
backend for templates (but not for plus concatenations).

#KT-36638 Fixed

compiler/testData/codegen/bytecodeText/kt5016.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36638 Use 'java/lang/StringBuilder.append (C)Ljava/lang/StringBuilder;' when appending single character in JVM_IR
-
 // KT-5016 wrong StringBuilder append method invoked
 class kt5016 {
     fun f1(name : String) : String {
@@ -10,4 +7,6 @@ class kt5016 {
 
 // 0 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/Object;\)Ljava/lang/StringBuilder
 // 2 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.toString