[AArch64][LoadStoreOptimizer] Turn a test into an assert. NFC.

At this point the given Opc must be valid, otherwise we should
not look for a matching pair to form paired load or store.

Thanks to Chad to point out this piece of code!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244366 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed