[m-dev.] For review: code generator improvements

[Thomas Charles CONWAY]

> 
> Tom, pleae review this.
> 
> Slight rearrangement of the data structures of the code generator, to allow
> more flexibility in code generation. The rearrangement moves the stack_slots
> information (mapping vars to their stack slots if any) and follow_vars
> information (mapping vars to the location preferred for them by future code)
> from code_info to code_exprn. This allows the predicates in code_exprn to
> make use of this information.
> 
> As a result of these changes, the code generator now emits 110 fewer lines
> of C code for the compiler (478 lines are replaced by 368). There is no
> discernible impact on the memory requirements or running time of the compiler
> itself.
> 
> code_exprn:
> 	Add the two fields to the exprn_info data structure.
> 
> 	Several predicates in code_exprn now evaluate variables directly
> 	into their preferred location, instead of a random register.
> 
> code_info:
> 	Remove the two fields from the code_info data structure.
> 

This diff looks fine.

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