[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

compiler/testData/codegen/bytecodeText/kt5016int.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 kt5016int {
     fun f1(num : Int) : String {
@@ -11,4 +8,6 @@ class kt5016int {
 // 0 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/Object;\)Ljava/lang/StringBuilder
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)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

compiler/testData/codegen/bytecodeText/kt5016intOrNull.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 kt5016intOrNull {
     fun f1(num : Int?) : String {
@@ -10,4 +7,6 @@ class kt5016intOrNull {
 
 // 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/Object;\)Ljava/lang/StringBuilder
 // 1 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

compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate.kt

@@ -0,0 +1,6 @@
+fun test(s: String, i: Int) = "${"x"}${s}${" "}${i}${"y"}"
+
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append

compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringInTemplate_2.kt

@@ -0,0 +1,6 @@
+fun test(s: String, i: Int) = "x${s} ${i}y"
+
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append

compiler/testData/codegen/bytecodeText/stringOperations/useAppendCharForOneCharStringUsingPlus.kt

@@ -0,0 +1,16 @@
+fun test(s: String, i: Int) = "x" + s + " " + i + "y"
+
+// The IR is equivalent for this test and "useAppendCharForOneCharStringInTemplate*.kt" because there is an optimization for 1-length
+// string literals in any string concatenation, whether using templates or + operator (see JvmStringConcatenationLowering).
+// However, for the non-IR backend, `append(String)` will still be used for these 1-length strings.
+
+// JVM_TEMPLATES
+// 4 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append
+
+// JVM_IR_TEMPLATES
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(Ljava/lang/String;\)Ljava/lang/StringBuilder
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.append \(I\)Ljava/lang/StringBuilder
+// 3 INVOKEVIRTUAL java/lang/StringBuilder.append \(C\)Ljava/lang/StringBuilder
+// 5 INVOKEVIRTUAL java/lang/StringBuilder.append