+ {- Case2: shift length is complex (non-immediate)
+ * y must go in %ecx.
+ * we cannot do y first *and* put its result in %ecx, because
+ %ecx might be clobbered by x.
+ * if we do y second, then x cannot be
+ in a clobbered reg. Also, we cannot clobber x's reg
+ with the instruction itself.
+ * so we can either:
+ - do y first, put its result in a fresh tmp, then copy it to %ecx later
+ - do y second and put its result into %ecx. x gets placed in a fresh
+ tmp. This is likely to be better, becuase the reg alloc can
+ eliminate this reg->reg move here (it won't eliminate the other one,
+ because the move is into the fixed %ecx).
+ -}