NFC: Rewrite tests so that they're actually testing something.

The code that tests if two functions are compatible to inline does a
compatibility check and then combines that with
`&& hasPossibleIncompatibleOps(..)`.

But `hasPossibleIncompatibleOps` always returns `false` for a function that
merely does a regular function call, which meant that many of the tests
weren't testing what they were supposed to test. By choosing a call to an
intrinsic that is sensitive to streaming-mode (i.e. llvm.vscale()), the
function will always return true, making the tests more sensible.

Author: sdesmalen-arm