[AArch64][GlobalISel] Select XRO addressing mode with wide immediates

Port the wide immediate case from AArch64DAGToDAGISel::SelectAddrModeXRO.

If we have a wide immediate which can't be represented in an add, we can end up
with code like this:

```
mov  x0, imm
add x1, base, x0
ldr  x2, [x1, 0]
```

If we use the [base, xN] addressing mode instead, we can produce this:

```
mov  x0, imm
ldr  x2, [base, x0]
```

This saves 0.4% code size on 7zip at -O3, and gives a geomean code size
improvement of 0.1% on CTMark.

Differential Revision: https://reviews.llvm.org/D84784