[AArch64][GlobalISel] Change store value type from p0 -> s64 to import patterns

Similar to the G_PTR_ADD + G_LOAD twiddling we do in `preISelLower`.

The imported patterns expect scalars only, so they can't handle things like

```
 G_STORE %ptr1, %ptr2
```

To get around this, use s64 instead.

(This probably makes a good portion of the manual selection code for G_STORE
dead.)

This is a 0.2% geomean code size improvement on CTMark at -Os.

(Best is consumer-typeset @ -0.7%)

Differential Revision: https://reviews.llvm.org/D95908