Fold shufps(hop(x,y),hop(z,w)) -> permute(hop(x,z)) - this is very similar to the equivalent unpack fold.
I did start trying to convert foldShuffleOfHorizOp to handle generic shuffle masks but we're relying on a lot of special cases at the moment.
See if we can remove the shuffle by resorting a HOP chain so that the HOP args are pre-shuffled.
This initial version just handles (the most common) v4i32/v4f32 hadd/hsub reduction patterns - future work can extend this to v8i16 types plus PACK chains (2f64 HADD/HSUB should already be handled in the half-lane combine code later on).
These are often generated when building a vector from the reduction sums of independent vectors.
I've implemented some typical patterns from various v4f32/v4i32 based off current codegen emitted from the vectorizers, although these tests are more about tweaking some hadd style backend folds to handle whatever the vectorizers/vectorcombine throws at us...
