[AArch64][GlobalISel] Refactor + improve CMN, ADDS, and ADD emit functions

These functions were extremely similar:

- `emitADD`
- `emitADDS`
- `emitCMN`

Refactor them a little, introducing a more generic `emitInstr` function to
do most of the work.

Also add support for the immediate + shifted register addressing modes in each
of them.

Update select-uaddo.mir to show that selecing ADDS now supports folding
immediates + shifts. (I don't think this can impact CMN, because the CMN checks
require a G_SUB with a non-constant on the RHS.)

This is around a 0.02% code size improvement on CTMark at -O3.

Differential Revision: https://reviews.llvm.org/D87529