[JS IR] Implement MoveTemporaryVariableDeclarationToAssignment optimization

js/js.translator/testData/box/jsAstOptimizations/tempVarDeclOnAssignment.kt

@@ -0,0 +1,10 @@
+inline fun jsUpper(c: Char) = c.toString().asDynamic().toUpperCase().unsafeCast<String>()
+
+inline fun upper(c: Char): Char = jsUpper(c)[0]
+
+// EXPECT_GENERATED_JS: function=test expect=tempVarDeclOnAssignmentTest.js
+fun test(a: Char, b: Char): String {
+    return "${upper(a)}${upper(b)}"
+}
+
+fun box() = test('o', 'k')

js/js.translator/testData/box/jsAstOptimizations/tempVarDeclOnAssignmentTest.js

@@ -0,0 +1,16 @@
+function test(a, b) {
+  // Inline function 'upper' call
+  // Inline function 'jsUpper' call
+  // Inline function 'kotlin.js.unsafeCast' call
+  // Inline function 'kotlin.js.asDynamic' call
+  var tmp$ret$2 = toString(a).toUpperCase();
+  var tmp$ret$3 = charSequenceGet(tmp$ret$2, 0);
+  var tmp = toString(tmp$ret$3);
+  // Inline function 'upper' call
+  // Inline function 'jsUpper' call
+  // Inline function 'kotlin.js.unsafeCast' call
+  // Inline function 'kotlin.js.asDynamic' call
+  var tmp$ret$6 = toString(b).toUpperCase();
+  var tmp$ret$7 = charSequenceGet(tmp$ret$6, 0);
+  return tmp + toString(tmp$ret$7);
+}