Fix KNPE caused by optimizations in control-flow analysis

The problem is that when performing full analysis we do it in
a backward order while result for trivial vals is filled
in a forward one.

It turns out that reversedInstuctions might return a superset of
forward traversed instructions, e.g. in case of dead code in lambda.

At the same time result for trivial vals is constant
for any instruction, thus we can just return its constant value
and use it in the full analysis

 #KT-20895 Fixed

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/returnInDeadLambda.kt

@@ -0,0 +1,11 @@
+
+inline fun myRun(b: () -> Unit) = b()
+
+fun foo() {
+    var <!UNUSED_VARIABLE!>a<!>: Int
+    return
+
+    <!UNREACHABLE_CODE!>myRun {
+        return
+    }<!>
+}

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/returnInDeadLambda.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(): kotlin.Unit
+public inline fun myRun(/*0*/ b: () -> kotlin.Unit): kotlin.Unit