A truncate is considered saturated if no additional conversion is
required between the target and return values. If the target is
saturated when attempting to truncate from a vector, there is an
opportunity to optimize it.

Previously, each architecture had its own attempt at optimization,
leading to redundant code. This patch implements common logic by
introducing three new ISDs:

 `ISD::TRUNCATE_SSAT_S`: When the operand is a signed value and
 the range of values matches the range of signed values of the
 destination type.

 `ISD::TRUNCATE_SSAT_U`: When the operand is a signed value and
 the range of values matches the range of unsigned values of the
 destination type.

 `ISD::TRUNCATE_USAT_U`: When the operand is an unsigned value and
 the range of values matches the range of unsigned values of the
 destination type.

These ISDs indicate a saturated truncate.

Fixes llvm#85903

Add support for saturated truncate with the following changes:

- Add action to Legal for types v8i16, v4i32, and v2i64
- Implement `isTypeDesirableForOp` to check for truncate conversions
- Add patterns for saturated truncate of supported types

Add support for saturated truncate by implementing the following changes:

- Add `TRUNCATE_[SU]SAT_[SU]` to the Action target of `TRUNCATE`
- Add `TRUNCATE_[SU]SAT_[SU]` to the TargetLowering target of `TRUNCATE`
- Convert `TRUNCATE_SSAT_S` to `TRUNCATE_VECTOR_VL_SSAT`
- Convert `TRUNCATE_[SU]SAT_U` to `TRUNCATE_VECTOR_VL_USAT`