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

[Yuta Mukai via llvm-commits]

ytmukai wrote:

A MachineMemOperand with UnknownSize causes the following assertion failure on AArch64 backend.
https://godbolt.org/z/j7qs6x4Pv

The value is used to initialize Align, which is the immediate cause of the problem.

https://github.com/llvm/llvm-project/blob/ca1da36aec963c5504ae7ae2e834b37856c476db/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp#L2355

https://github.com/llvm/llvm-project/blob/364f781344e11d6a781ebdd0a4d9689bc9c51cfb/llvm/include/llvm/Support/Alignment.h#L78

However, this modification might be avoided because dropping MachineMemOperand will also result in the loss of alias information.

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