stored bytes in Alias information

stored bytes in Alias information

Stack slots may have padding
bytes, and if we want to have
precise liveness information
it's important that we are able
to tell them apart.

This patch extends fillalias()
to remember for every slot
what bytes were ever assigned.
In case the slot address does
not escape we know that only
these bytes matter.

To save space, we only store
this information if the slot
size is less than or equal to
NBit.

The Alias struct was reworked
a bit to save some space. I am
still not very satisfied with
its layout though.