[AArch64][GlobalISel] Share address mode selection code for memops

We were missing support for the G_ADD_LOW + ADRP folding optimization in the
manual selection code for G_LOAD, G_STORE, and G_ZEXTLOAD.

As a result, we were missing cases like this:

```
@foo = external hidden global i32*
define void @baz(i32* %0) {
store i32* %0, i32** @foo
ret void
}
```

https://godbolt.org/z/16r7ad

This functionality already existed in the addressing mode functions for the
importer. So, this patch makes the manual selection code use
`selectAddrModeIndexed` rather than duplicating work.

This is a 0.2% geomean code size improvement for CTMark at -O3.

There is one code size increase (0.1% on lencod) which is likely because
`selectAddrModeIndexed` doesn't look through constants.

Differential Revision: https://reviews.llvm.org/D87397