This patch adds the following DAGCombines, which apply if isVectorLoadExtDesirable() returns true:
- fold (and (masked_gather x)) -> (zext_masked_gather x)
- fold (sext_inreg (masked_gather x)) -> (sext_masked_gather x)
LowerMGATHER has also been updated to fetch the LoadExtType associated with the
gather and also use this value to determine the correct masked gather opcode to use.