[SLP][AArch64] Extend extracts-from-scalarizable-vector.ll test for cmp cost testing. NFC

See D153507. The existing test is over-simplified, as written it should have
been simpified prior to SLP vectorization. I have left it as-is to ensure the
crash it was protecting against doesn't arise again. A new test with valid
inputs is also added to show the incorrect costs of alt cmp vectorization.