implemented RReil grammar is cannot express sizes for comparisons

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The current RReil grammar does not match the one published in the APLAS paper. 
We use a very unintuitive short-cut that avoids storing the size of a 
comparison in the ```sexpr```.

The statement

{{{
FLAGS :=1 x <=u8 y
}}}

that should assign the 1-bit result of the 8-bit unsigned comparison in FLAGS 
uses the size field of the assignment to store the size of the comparison (i.e. 
internally the assignment above is stored as

{{{
FLAGS :=8 x <=u y
}}}

and the comparison has no size field). This is not only counter-intuitive but 
it prevents us from writing

{{{
cbranch x <=u8 y ? tgt1 : tgt2
}}}

since ```cbranch``` does not have a size field where we could store 8. Thus, we 
should have a size field in the comparison rule of ```sexpr`` and use the 
intuitive semantics in that we store 1 for the size of the assignment for 
comparisons.

Original issue reported on code.google.com by [email protected] on 14 Nov 2014 at 6:53