[AArch64][Fix] LdSt optimization generate premature stack-popping

Summary:
When moving add and sub to memory operand instructions,
aarch64-ldst-opt would prematurally pop the stack pointer,
before memory instructions that do access the stack using
indirect loads.
e.g.
```
int foo(int offset){
    int local[4] = {0};
    return local[offset];
}
```
would generate:
```
sub     sp, sp, #16            ; Push the stack
mov     x8, sp                 ; Save stack in register
stp     xzr, xzr, [sp], #16    ; Zero initialize stack, and post-increment, making it invalid
------ If an exception goes here, the stack value might be corrupted
ldr     w0, [x8, w0, sxtw #2]  ; Access correct position, but it is not guarded by SP
```

Reviewers: fhahn, foad, thegameg, eli.friedman, efriedma

Reviewed By: efriedma

Subscribers: efriedma, kristof.beyls, hiraditya, danielkiss, llvm-commits, simon_tatham

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75755