[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/firProblems/functionLiteralGenericSignature.fir.kt.txt

@@ -0,0 +1,53 @@
+val unitFun: Function0<Unit>
+  field = local fun <anonymous>() {
+    return Unit
+  }
+
+  get
+
+val intFun: Function0<Int>
+  field = local fun <anonymous>(): Int {
+    return 42
+  }
+
+  get
+
+val stringParamFun: Function1<String, Unit>
+  field = local fun <anonymous>(x: String) {
+    return Unit
+  }
+
+  get
+
+val listFun: Function1<List<String>, List<String>>
+  field = local fun <anonymous>(l: List<String>): List<String> {
+    return l
+  }
+
+  get
+
+val mutableListFun: Function1<MutableList<Double>, MutableList<Int>>
+  field = local fun <no name provided>(l: MutableList<Double>): MutableList<Int> {
+    return CHECK_NOT_NULL<Nothing>(arg0 = null)
+  }
+
+  get
+
+val funWithIn: Function1<Comparable<String>, Unit>
+  field = local fun <no name provided>(x: Comparable<String>) {
+  }
+
+  get
+
+val extensionFun: @ExtensionFunctionType Function1<Any, Unit>
+  field = local fun Any.<no name provided>() {
+  }
+
+  get
+
+val extensionWithArgFun: @ExtensionFunctionType Function2<Long, Any, Date>
+  field = local fun Long.<no name provided>(x: Any): Date {
+    return Date()
+  }
+
+  get