[FIR] Copy implications from previous data flows when there is only one

Postponed lambdas introduce a host of challenges in data-flow analysis.
While inheriting type statements was disabled while these challenges are
being considered, we must still copy type statements from non-postponed
lambda edges. It seems the same is true for implications.

Implications are copied from previous flows when there is only a single
previous flow. That is because it never seemed to be required based on
test results. However, a recent test case revealed that copying is
required when there are multiple previous flows, but only one flow is
from a non-postponed-lambda node.

Combining implications from multiple non-postponed-lambda nodes did not
have an impact on test results, so until such a test case can be
created, the overhead of calculating common implications from multiple
flows will be avoided.

^KT-63351 Fixed

compiler/testData/diagnostics/tests/smartCasts/safecalls/longChain.fir.kt

@@ -1,6 +1,14 @@
-fun calc(x: List<String>?) {
+// WITH_STDLIB
+// ISSUE: KT-63351
+
+fun test1(x: List<String>?) {
     // x should be non-null in arguments list, despite of a chain
     x?.subList(0, x.size)?.
        subList(0, x.size)?.
        get(x.size)
 }
+
+fun test2(x: List<String>?) {
+    x?.filter { true }!!.size
+    x.size
+}

compiler/testData/diagnostics/tests/smartCasts/safecalls/longChain.kt

@@ -1,6 +1,14 @@
-fun calc(x: List<String>?) {
+// WITH_STDLIB
+// ISSUE: KT-63351
+
+fun test1(x: List<String>?) {
     // x should be non-null in arguments list, despite of a chain
     x?.subList(0, <!DEBUG_INFO_SMARTCAST!>x<!>.size)?.
        subList(0, <!DEBUG_INFO_SMARTCAST!>x<!>.size)?.
        get(<!DEBUG_INFO_SMARTCAST!>x<!>.size)
 }
+
+fun test2(x: List<String>?) {
+    x?.filter { true }!!.size
+    <!DEBUG_INFO_SMARTCAST!>x<!>.size
+}

compiler/testData/diagnostics/tests/smartCasts/safecalls/longChain.txt

@@ -1,3 +1,4 @@
 package
 
-public fun calc(/*0*/ x: kotlin.collections.List<kotlin.String>?): kotlin.Unit
+public fun test1(/*0*/ x: kotlin.collections.List<kotlin.String>?): kotlin.Unit
+public fun test2(/*0*/ x: kotlin.collections.List<kotlin.String>?): kotlin.Unit