- x & 3  => x % SC_BITS
- x >> 2 => x / SC_BITS
(after everything is converted to unsigned this should result in the
 exact same code as before).

This fixes opt/doms_segfault2.c.

We cannot delete the Store if its address aliases with the source of the
CopyB.

This fixes opt/loadstore3.c.

Code placement cannot handle them appropriately.

This fixes backend/sparc_keep.c.