[llvm] [MachinePipeliner] Fix MachineMemOperand to be cleared when update fails (PR #80841)

[Yuta Mukai via llvm-commits]

ytmukai wrote:

@davemgreen Thanks for your comments, I've reconsidered the load/store code should be modified.
It guarantees that the address of ldp/stp is aligned to twice the element size by MachineMemOperand if it needs to be. However, it assumes that the alignment is guaranteed if MachineMemOperand does not exist or has an invalid value, which appears to be a problem.
I will propose a modification that would determine it more conservatively.

https://github.com/llvm/llvm-project/blob/ca1da36aec963c5504ae7ae2e834b37856c476db/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp#L2347-L2362

https://github.com/llvm/llvm-project/pull/80841