[SLPVectorizer] Pre-commit a test for D85759

8cc911fa refactored the `getIntrinsicInstrCost` function and was
meant to be a nonfunctional change, but it accidentally changed how
costs were calculated in the SLP vectorizer, which regressed
WebAssembly codegen and resulted in a downstream bug report at
https://github.com/emscripten-core/emscripten/issues/11449.

The fix for this regression is in D85759, and this patch just
pre-commits the test from that patch to demonstrate the regressed
behavior first.