JVM_IR: optimize more if-null chains

1. consider reads of fields from the same file "stable" just like
    functions, i.e. assume their nullability information is correct
 2. apply if-null fusion repeatedly until the subject is no longer a
    nested if-null expression

compiler/testData/codegen/bytecodeText/nullCheckOptimization/safeCallAndElvisChains.kt

@@ -0,0 +1,18 @@
+class A(val x: String) {
+    fun y() = x
+    // Both `x` and `y()` assumed to respect their nullability information,
+    // so only `a` and `b` need to be checked.
+    fun foo1(a: A?, b: A?) = a?.x ?: b?.x ?: x       // if (a == null) if (b == null) x else b.x else a.x
+    fun foo2(a: A?, b: A?) = a?.y() ?: b?.y() ?: y() // if (a == null) if (b == null) y() else b.y() else a.y()
+}
+
+// JVM_TEMPLATES
+// Optimization not implemented
+// 8 IFNULL
+// 0 IFNONNULL
+// 2 ACONST_NULL
+
+// JVM_IR_TEMPLATES
+// 0 IFNULL
+// 4 IFNONNULL
+// 0 ACONST_NULL