[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/diagnostics/testsWithStdLib/tryCatch/assignTry.fir.kt

@@ -5,7 +5,7 @@ class ExcA : Exception()
 class ExcB : Exception()
 
 fun test2() {
-    val s: String? = <!INITIALIZER_TYPE_MISMATCH, TYPE_MISMATCH!>try {
+    val s: String? = <!INITIALIZER_TYPE_MISMATCH!>try {
         ""
     }
     catch (e: ExcA) {