ALIAS instructions are messed up?

[Joern Rennecke]

> Whether to disallow alias insns entirely is a separate question. Dunno.
> [We could still keep it for macro-insns.
> And I realize there are a few ports that currently use it, or at  
> least specify > it.]

Currently, macro-instructions can't be used in all instances where
it should be natural to use them.
When you want to set a multi-ifield to a constant, you end up with
the entire costant being added in in the bit position of each of
constituent simple bitfields.

By adding an ALIAS instruction by hand, you can at least work around
the problem by setting only simple bitfields to constants.