[AArch64] Use SUBXrx64 for dynamic stack to refer to sp

When we lower dynamic stack, we need to substract pattern `x15 << 4`  from sp.
Subtract instruction with arith shifted register(SUBXrs) can't refer to sp. So for now we need two extra mov like:

```
mov x0, sp
sub x0, x0, x15, lsl #4
mov sp, x0
```
If we want to refer to sp in subtract instruction like this:
```
sub	sp, sp, x15, lsl #4
```
We must use arith extended register version(SUBXrx).
So in this patch when we find sub have sp operand on src0, try to select to SubXrx64.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D129932