fix JS optimizer fail on ES6 spread operator

[pavelsavara]

fixes#12543

[sbc100]

Would be good to add a test somewhere. @kripken probably knows where the best place might be

[kripken]

lgtm too, thanks! For a test, please add something minimal here:

https://github.com/emscripten-core/emscripten/blob/main/test/optimizer/JSDCE.js

(expected output goes here: https://github.com/emscripten-core/emscripten/blob/main/test/optimizer/JSDCE-output.js )

[pavelsavara]

I don't have the tests running on my local box, so I will make test iterations on the CI. Sorry for the noise.

[kripken]

Running locally, this is the diff from --rebaseline:

diff --git a/test/optimizer/JSDCE-output.js b/test/optimizer/JSDCE-output.js
index 89c4957f7..480f0396b 100644
--- a/test/optimizer/JSDCE-output.js+++ b/test/optimizer/JSDCE-output.js@@ -14,6 +14,19 @@ function g(a) {
Module["g"] = g;
+const sx = {+ a: 1+};++const sar = [ 1, 2, 3 ];++Module["spread"] = {+ b: 2,+ ...sx+};++Module["spread2"] = [ ...sar, 4, 5, 6 ];+
function h(a) {
return a + 1;
}

[kripken]

Committed, should pass I hope...