This implementation avoids two unneeded MOVs for each 64-bit
component. One was done in the old shuffle, to avoid cases of
src/dst overlap but this is not the case. And the removed MOV
was already being being done in the shuffle.
Copy propagation wasn't able to remove them because shuffle
destination values are defined with partial writes because they
have stride == 2.
v2: Reword commit log summary (Jason Ekstrand)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
152bffb69b