[AArch64][GlobalISel] Select arith extended add/sub in manual selection code

The manual selection code for add/sub was not checking if it was possible to
fold in shifts + extends (the *rx opcode variants).

As a result, we could never select things like

```
cmp x1, w0, uxtw #2
```

Because we don't import any patterns for compares.

This adds support for the arithmetic shifted register forms and updates tests
for instructions selected using `emitADD`, `emitADDS`, and `emitSUBS`.

This is a 0.1% geomean code size improvement on SPECINT2000 at -Os.

Differential Revision: https://reviews.llvm.org/D91207

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