[AArch64][GlobalISel] Select G_SADDO and G_SSUBO

We didn't have selector support for these.

Selection code is similar to `getAArch64XALUOOp` in AArch64ISelLowering. Similar
to that code, this returns the AArch64CC and the instruction produced. In SDAG,
this is used to optimize select + overflow and condition branch + overflow
pairs. (See `AArch64TargetLowering::LowerBR_CC` and
`AArch64TargetLowering::LowerSelect`)

(G_USUBO should be easy to add here, but it isn't legalized right now.)

This also factors out the existing G_UADDO selection code, and removes an
unnecessary check for s32/s64. AFAIK, we shouldn't ever get anything other than
s32/s64. It makes more sense for this to be handled by the type assertion in
`emitAddSub`.

Differential Revision: https://reviews.llvm.org/D92610

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