[AArch64] Verify ldp/stp alignment stricter (#83948)

When ldp-aligned-only/stp-aligned-only is specified, modified to cancel
ldp/stp transformation if MachineMemOperand is not present or the access
size is unknown.
In the previous implementation, the test passed when there was no
MachineMemOperand. Also, if the size was unknown, an incorrect value was
used or an assertion failed. (But actually, if there is no
MachineMemOperand, it will be excluded from the target by
isCandidateToMergeOrPair() before reaching the part.)

A statistic NumFailedAlignmentCheck is added. NumPairCreated is modified
so that it only counts if it is not cancelled.