[m-dev.] diff: mostly-uniq modes changes

[Thomas Charles CONWAY]

Fergus Henderson, you write:
> 
> A few small changes towards supporting backtrackable destructive update
> better.
> 
> library/mercury_builtin.m:
> 	Add new insts mdi/1 and mostly_dead/0.
> 

Possibly, we should add another inst: slightly_alive as an
equivalence to mostly_dead.

Thomas
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Thomas Conway               				      conway at cs.mu.oz.au
AD DEUM ET VINUM	  			      Every sword has two edges.