Useless reloads between calls

[xclerc]

The split/rename preprocessing phase of
the register allocators tries to move spills
and reloads, and delete them when they
cancel each other out. Unfortunately, we
currently move the instructions only in
the case of simple sequence of blocks,
which is too limited to delete all useless
spills and reloads.

We should try to improve that phase with
more aggressive code motion, or change
the initial placement of spills/reloads to
not only use the information about liveness
but also whether the value will be used
before the next destruction point.