Rename tests with diagnostics on JVM backend

Remove the "_ir" suffix. It was originally used to distinguish tests on
the JVM IR backend from the old JVM backend ("_old"), but the latter are
removed now.

compiler/testData/diagnostics/testsWithJvmBackend/duplicateJvmSignature/specialNames/instance.kt

@@ -0,0 +1,10 @@
+// FIR_IDENTICAL
+// TARGET_BACKEND: JVM_IR
+
+object O {
+    val INSTANCE: O = null!!
+}
+
+<!CONFLICTING_JVM_DECLARATIONS!>object O2<!> {
+    <!CONFLICTING_JVM_DECLARATIONS!>lateinit var INSTANCE: O2<!>
+}