[llvm] [LLVM] Add `llvm.masked.compress` intrinsic (PR #92289)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> Is there a place to put the duplicated code between the DagTypeLegalizer and the VectorLegalizer?

TargetLowering has a bunch of expand* routines.

> SplitVecOperand or WidenResultVector. Maybe someone could point me in the right direction here, as none of my test cases fall into these paths.

Suggested types to test with:

- `<3 x i32>`.
- `<vscale x 4 x double>`

It should be possible to exercise all forms of result legalization.  Some forms of operand legalization might be impossible to hit: we legalize results before operands, so the possible operand types are restricted.

https://github.com/llvm/llvm-project/pull/92289