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
56fcd4ea8d