[FIR] Implement new completion mode calculator

Note that `testDelegates` now fails due to KT-37638 and
    `testSimpleIn` fails due to problems with type parameters
    of inner classes

compiler/testData/ir/irText/expressions/whenUnusedExpression.fir.txt

@@ -3,7 +3,7 @@ FILE fqName:<root> fileName:/whenUnusedExpression.kt
     VALUE_PARAMETER name:b index:0 type:kotlin.Boolean
     VALUE_PARAMETER name:i index:1 type:kotlin.Int
     BLOCK_BODY
-      WHEN type=kotlin.Int? origin=IF
+      WHEN type=kotlin.Int origin=IF
         BRANCH
           if: GET_VAR 'b: kotlin.Boolean declared in <root>.test' type=kotlin.Boolean origin=null
           then: BLOCK type=kotlin.Int origin=WHEN