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Another small GCSE speedup


GCSE maintains a pair of lists for memory modifications for each block. 
One contains the original memory reference, the other the canonicalized
memory reference.

To avoid the sometimes significant overhead of clearing all the lists
GCSE also maintains a bitmap of blocks which have non-empty memory 
modification lists.  Actually, GCSE maintains _two_ bitmaps of blocks,
one for the main list, one for the canonicalized list.

That's rather silly since the two bitmaps will always be identical.
This patch kills the second bitmap.

This results in a small (< .5%), but consistently measurable speedup.

Bootstrapped and regression tested on i686-pc-linux-gnu.


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