Autoincrement addressing modes

[Joern Rennecke]

> At this stage I'm interested in what other common forms should be
> supported.

The SH doesn't have any exotic update forms, but it has another
quirk in autoincrement that gcc doesn't handle efficiently:
POST_INC is only available for reading, while PRE_DEC is only
available for writing.

There are some places in the compiler that just test POST_INC / PRE_DEC
(or more generally HAVE_AUTO_INC) to check if an autoincrement might
be generated in a later stage.  It would be nice if these would
be updated not only to handle the new address updates properly, but also
only speculate the appropriatenes of an address update depending on
the direction of the memory access.