Filter nullables after lifting union over spread

Previously, null/undefined were removed upfront, which is incorrect. Now
nullable types are removed with other non-primitives after union is
lifted over spread. This produces unions, which are larger, but more correct.

Author: sandersn

src/compiler/checker.ts

@@ -7632,11 +7632,9 @@ namespace ts {
             if (left.flags & TypeFlags.Any || right.flags & TypeFlags.Any) {
                 return anyType;
             }
-            left = filterType(left, t => !(t.flags & TypeFlags.Nullable));
             if (left.flags & TypeFlags.Never) {
                 return right;
             }
-            right = filterType(right, t => !(t.flags & TypeFlags.Nullable));
             if (right.flags & TypeFlags.Never) {
                 return left;
             }