LoopVectorizer is aware when a target can replace a scalable frem
instruction with a vector library call for a given VF and it returns the
relevant cost. Otherwise, it returns an invalid cost (as previously).
Add test that check costs on AArch64, when there is no vector library
available and when there is (with and without tail-folding).
NOTE: Invoking CostModel directly (not through LV) would still return
invalid costs.
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