Made IsAssignableVisitor ignore Poison values

This fixes some spurious errors being reported by DSLParser.

Change-Id: I824896b88c2d208aac13cc8f597a2f949b831196
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443057
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>

src/sksl/SkSLAnalysis.cpp

@@ -336,6 +336,9 @@ public:
                 this->visitExpression(*expr.as<IndexExpression>().base());
                 break;
 
+            case Expression::Kind::kPoison:
+                break;
+
             default:
                 fErrors->error(expr.fOffset, "cannot assign to this expression");
                 break;