[m-dev.] for review: fix a bug in value numbering reported by Simon

[Simon Taylor]

> compiler/vn_block.m:
> 	Restore a previously commented block of code, whose job is to force
> 	the flushing of any stack slot that holds the saved value of a nondet
> 	control register or stack slot. The rationale for this code used
> 	to be the handling of cuts in nondet code; now it is the handling
> 	of hijackings.

Thanks Zoltan, commit away.

Simon.