[x86] Restore the bitcasts I removed when refactoring this to avoid

shifting vectors of bytes as x86 doesn't have direct support for that.

This removes a bunch of redundant masking in the generated code for SSE2
and SSE3.

In order to avoid the really significant code size growth this would
have triggered, I also factored the completely repeatative logic for
shifting and masking into two lambdas which in turn makes all of this
much easier to read IMO.

llvm-svn: 238637