unused: detect struct conversions involving generics

(cherry picked from commit d694aad)

unused/testdata/src/typeparams/typeparams.go

@@ -89,3 +89,17 @@ func fn11[T ~struct{ Field int }]() { // unused
 	// don't crash because of the composite literal
 	_ = T{Field: 42}
 }
+
+type convertGeneric1 struct { // used
+	field int // used
+}
+
+type convertGeneric2 struct { // used
+	field int // used
+}
+
+var _ = convertGeneric1{}.field // mark field as used
+
+func Fn12[T1 convertGeneric1, T2 convertGeneric2](a T1) { // used
+	_ = T2(a) // conversion marks T2.field as used
+}

unused/unused.go

@@ -1662,8 +1662,8 @@ func (g *graph) instructions(fn *ir.Function) {
 			case *ir.ChangeType:
 				// conversion type handled generically
 
-				s1, ok1 := typeutil.Dereference(instr.Type()).Underlying().(*types.Struct)
-				s2, ok2 := typeutil.Dereference(instr.X.Type()).Underlying().(*types.Struct)
+				s1, ok1 := typeutil.CoreType(typeutil.Dereference(instr.Type())).(*types.Struct)
+				s2, ok2 := typeutil.CoreType(typeutil.Dereference(instr.X.Type())).(*types.Struct)
 				if ok1 && ok2 {
 					// Converting between two structs. The fields are
 					// relevant for the conversion, but only if the