[FIR] Add equality constraint from expected type for some synthetic function calls

This fixes some cases where we infer some type variable inside one
of the branches to Nothing instead of the expected type because Nothing
appeared in some other branch.

Specifically, we add an equality instead of a subtype constraint during
completion of calls to synthetic functions for if/when, try and !!.
We don't do it when the call contains a (possibly nested) elvis or is
inside the RHS of an assignment.
Otherwise, we would prevent some smart-casts.

#KT-65882 Fixed

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

@@ -71,7 +71,7 @@ FILE fqName:<root> fileName:/implicitCastToTypeParameter.kt
       $receiver: VALUE_PARAMETER name:<this> type:<root>.Foo<T of <root>.<get-asT>>
       BLOCK_BODY
         RETURN type=kotlin.Nothing from='public final fun <get-asT> <T> (): T of <root>.<get-asT>? declared in <root>'
-          WHEN type=kotlin.Any? origin=IF
+          WHEN type=T of <root>.<get-asT>? origin=IF
             BRANCH
               if: TYPE_OP type=kotlin.Boolean origin=INSTANCEOF typeOperand=T of <root>.<get-asT>
                 GET_VAR '<this>: <root>.Foo<T of <root>.<get-asT>> declared in <root>.<get-asT>' type=<root>.Foo<T of <root>.<get-asT>> origin=null

compiler/testData/ir/irText/expressions/kt50028.fir.ir.txt

@@ -2,7 +2,7 @@ FILE fqName:<root> fileName:/kt50028.kt
   FUN name:test_1 visibility:public modality:FINAL <> () returnType:kotlin.String
     BLOCK_BODY
       RETURN type=kotlin.Nothing from='public final fun test_1 (): kotlin.String declared in <root>'
-        WHEN type=kotlin.Nothing origin=WHEN
+        WHEN type=kotlin.String origin=WHEN
           BRANCH
             if: CONST Boolean type=kotlin.Boolean value=true
             then: BLOCK type=kotlin.Unit origin=null