Fix FixedReg parser when it encounters constants with the high bit set.
Extend debugging a little.
Whoops: when doing subst_eq's backwards pass, the conditions you
generate apply to your predecessors but not to yourself.  Make sure we
realise that.
Give StartAtomic effects a MAI, jsut to make things a bit more uniform.
Use the StartAtomic MAI to remove a special case when building
cross-product machines.
The sub-word store support is a bit flakey.  Work around it by just
replacing every sub-word access with a read-modify-write cycle.
Extend realias to try to reduce the type of store operations, where
that's useful and safe.
Deadcode's complicated pointer/data split doesn't work any more.  Kill
it off.
Do a type reduction pass in deadcode elimination, as well.
Do a better job of the various rules for reducing expressions involving 0.
noExtend() definitely shoudn't be on globally.  Not sure why it is.
Turn it off.
Tidy up StackLayout in realias a bit.
Unbreak realias in the presence of multi-threaded machines.