JS: fix bad join order in Xss.qll

[erik-krogh]

I was looking for a performance regression in VSCode.
I'm quite sure I haven't found it yet, but I found this thing.

This is how the tuple counts looked before:

[2021-04-19 13:07:24] (3s) Tuple counts for Xss::DomBasedXss::JQueryHtmlOrSelectorArgument#f/1@a9a054:
                      14883760 ~4%     {2} r1 = SCAN TypeInference::AnalyzedNode::getAType_dispred#ff@staged_ext OUTPUT In.1, In.0 'this'
                      1579031  ~6%     {1} r2 = JOIN r1 WITH construct<TypeTag,4>@dom#InferredTypes::TTString#0#f ON FIRST 1 OUTPUT Lhs.1 'this'
                      30       ~2%     {2} r3 = JOIN r2 WITH jQuery::JQuery::MethodCall::interpretsArgumentAsHtml_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'this'
                      30       ~0%     {1} r4 = JOIN r3 WITH jQuery::JQuery::MethodCall::interpretsArgumentAsSelector_dispred#ff ON FIRST 2 OUTPUT Lhs.1 'this'

And with this change the max number of tuples is 30.

Evaluation looks good. (internal link, was partially done when I checked).