[JVM IR] Maintain KT-36625 bug compatibility between non-IR and IR

backends by removing IMPLICIT_NOTNULL casts from IrStringConcatenation
arguments.

Also fixed an issue where IrStringConcatenation can be lowered into
a null String instead of a literal "null" String if the lone argument
was a platform type String or String with enhanced nullability and the
value was null (e.g., "${FromJava.nullPlatformString()}").

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

@@ -1,8 +1,6 @@
-// IGNORE_BACKEND: JVM
-
 fun f(s: String) = "$s"
 
 fun g(s: String?) = "$s"
 
-// 1 valueOf
+// 2 valueOf
 // 0 NEW java/lang/StringBuilder