[llvm] [RISCV] Support `llvm.masked.compressstore` intrinsic (PR #83457)

[Craig Topper via llvm-commits]

================
@@ -1620,3 +1620,15 @@ bool RISCVTTIImpl::isLSRCostLess(const TargetTransformInfo::LSRCost &C1,
                   C2.NumIVMuls, C2.NumBaseAdds,
                   C2.ScaleCost, C2.ImmCost, C2.SetupCost);
 }
+
+bool RISCVTTIImpl::isLegalMaskedCompressStore(Type *DataTy, Align Alignment) {
----------------
topperc wrote:

The check being asked for is that fixed vectors are supported. Not that the specific type is legal. We still have a master disable for fixed vectors in the backend by setting -rvv-vector-bits-min=0.

```
    // Only support fixed vectors if we know the minimum vector size.
    if (DataTypeVT.isFixedLengthVector() && !ST->useRVVForFixedLengthVectors())
      return false;
```

https://github.com/llvm/llvm-project/pull/83457