[GlobalISel] Combine (G_*ADDO x, 0) -> x + no carry out

[GlobalISel] Combine (G_*ADDO x, 0) -> x + no carry out

Similar to the G_*MULO change.

The code for checking if a constant is legal/pre-legalize is shared between
these, and is kind of hairy. So, factor it out into a new function:
`isConstantLegalOrBeforeLegalizer`.

To make the refactoring clean, further refactor `isLegalOrBeforeLegalizer` into
a wrapper for two functions:

- `isPreLegalize`
- `isLegal`

This is a bit easier to read in general.

https://godbolt.org/z/KW7oszP1o

Differential Revision: https://reviews.llvm.org/D118655