[test][AArch64] Add a test case for D121180 NFC

Now, perform last active true vector combine only where
we're extracting from a flag-setting operation. But in
fact, the last active extracting will output LASTB + WHILELS,
and the WHILELS itself is a flag-setting operation, so
precommit this case to test the potentially further optimization.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D122453