move more slots to vregs

[graydon]

Currently we have a force-to-memory treatment that's too aggressive (Alias pins
any slot that's ever aliased, at all). As well, layout won't move any
exteriors or alias slots into vregs. This is all far too strong; we have (in
the meantime) grown support for temporarily forcing an explicit spill from
trans, which makes us more able to vreg-ify word-sized values.

So .. exploit this fact and move more stuff to vregs.

[graydon]

WONTFIX (not required for bootstrapping)