[TargetLowering] Prevent expandVectorFindLastActive from creating illegal vector types during vector op legalization. (#190914) This code needs to create a step vector but we only have a mask vector type. If the step vector is too large it might not be an MVT. This causes the getSimpleVT() call for getTypeAction to fail. We can replace that with the EVT version of getTypeAction, but we'll still fail trying to legalize the vselect. The getOperationAction query will return Expand for non-simple VTs. ExpandVSELECT will try to unroll the vselect which will fail for scalable vectors. We could hack that to not unroll scalable vectors, but that would be a hack. To fix this, split the FIND_LAST_ACTIVE into two if the step vector needs to be split. Those will recursively legalize and eventually arrive at a size we can create a valid step vector for. One existing test changes because it created an illegal type which happened to still be an MVT. This allowed getOperationAction to return Legal, even though the type isn't legal. Fixes the assertion mentioned in #187458. Assisted-by: Claude Sonnet 4.5
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