SSE2: use unaligned intrinsics for loads

This seems to work around what I believe is a nasty bug in ghci's runtime
linker, in which it does not honour data alignment requirements.

--HG--
extra : rebase_source : 9607086ed0da4f9d25d4d6641c0f08af9018ecee
Help gcc to find some static loop unrolling opportunities

(I was surprised it wasn't smart enough already!)

--HG--
extra : rebase_source : 47e57aecc86eda911460d7cf260025dd61135019