[llvm] [AArch64] Move LDR_PXI from isStoreToStackSlot to isLoadFromStackSlot (PR #65658)

[via llvm-commits]

weiguozhi wrote:

> Did this have any effect on codegen?

I haven't  observed. 
But it impacts my work on codegen, more specifically when I try to consider spill/reload in MachineCopyPropagation pass.
These two functions are also used in several other codegen passes.

https://github.com/llvm/llvm-project/pull/65658