This canonicalizes fneg/fabs (shuffle X, poison, mask) -> shuffle
(fneg/fabs X), posion, mask
This undoes part of b331a7ebc1 and
a8f13dbdeb, but keeps the binary shuffle
case i.e. shuffle fneg, fneg, mask.
By pulling out the shuffle we bring it inline with the same
canonicalisation we perform on binary ops and intrinsics, which the
original commit acknowledges it goes in the opposite direction.
However nowadays VectorCombine is more powerful and can do more
optimisations when the shuffle is pulled out, so I think we should
revisit this. In particular we get more shuffles folded and can perform
scalarization.
d0c1ea928c