[X86] Fold ADD(VPMADDWD(X,Y),VPMADDWD(Z,W)) -> VPMADDWD(SHUFFLE(X,Z), SHUFFLE(Y,W))

[X86] Fold ADD(VPMADDWD(X,Y),VPMADDWD(Z,W)) -> VPMADDWD(SHUFFLE(X,Z), SHUFFLE(Y,W))

Merge addition of VPMADDWD nodes if each element pair doesn't use the upper element in each pair (i.e. its zero) - we can generalize this to either element in the pair if we one day create VPMADDWD with zero lower elements.

There are still a number of issues with extending/shuffling with 256/512-bit VPMADDWD nodes so this initially only works for v2i32/v4i32 cases - I'm working on removing all these limitations but there's still a bit of yak shaving to go.....