[GlobalISel] Support saturated truncate (#150219)
Implements combining and legalization of G_TRUNC_SSAT_S, G_TRUNC_SSAT_U,
and G_TRUNC_USAT_U, which where previously added to SDAG with the below
patterns:
```
truncate(smin(smax(x, C1), C2)) -> trunc_ssat_s(x)
truncate(smax(smin(x, C2), C1)) -> trunc_ssat_s(x)
truncate(smax(smin(x, C), 0)) -> trunc_ssat_u(x)
truncate(smin(smax(x, 0), C)) -> trunc_ssat_u(x)
truncate(umin(smax(x, 0), C)) -> trunc_ssat_u(x)
truncate(umin(x, C)) -> trunc_usat_u(x)
```
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