liblzma: SHA-256: Optimize the way rotations are done.

liblzma: SHA-256: Optimize the way rotations are done.

This looks weird because the rotations become sequential,
but it helps quite a bit on both 32-bit and 64-bit x86:

  - It requires fewer instructions on two-operand
    instruction sets like x86.

  - It requires one register less which matters especially
    on 32-bit x86.

I hope this doesn't hurt other archs.

I didn't invent this idea myself, but I don't remember where
I saw it first.