When I moved combineLoopMAddPattern to an IR pass. I didn't match the behavior of canReduceVMulWidth that was used in the SelectionDAG version. canReduceVMulWidth just calls computeSignBits and assumes a truncate is always profitable. The version I put in IR just looks for constants and zext/sext. Though I neglected to check the number of bits in input of the zext/sext.
This patch adds a check for the number of input bits to the sext/zext. And it adds a special case for add/sub with zext/sext inputs which can be handled by combineTruncatedArithmetic. Match the original SelectionDAG behavior appears to be a regression in some cases if the truncate isn't removed and becomes pack and permq. So enabled only this specific case is the conservative approach.