[m-dev.] for review: bug-avoidance in value numbering

[Zoltan Somogyi]

> > value_number.m:
> > 	Do not attempt to optimize code sequences that manipulate tickets.
> > 	That code is buggy, and it is not worth fixing in this incarnation
> > 	of the value numbering optimization.
> 
> The code in the diff also disables optimization of code sequences
> that contain c_code, pragma_c, or block instructions.  Why?

There is already code that disables value numbering for entire procedures
if they contain c_code or pragma_c instructions, so disabling them again
for code fragments is double insurance. Block instructions are only introduced
in the post-value number pass, which is run after all invocations of
value numbering.

I will change the handling of all three to call error instead.

Zoltan.