PATCH to compute_flow_insensitive_aliasing for c++/27768

[Jason Merrill]

The bug here was that there were some may_aliases in one alias set that 
never made it into the bitmap representation, so when that set was 
merged into another in group_aliases they were lost and so the store to 
w.p looked dead.

Fixed by explicitly copying the may_aliases from the annotations into 
the bitmap rather than expecting them to be recreated.

Tested x86_64-pc-linux-gnu, applied to trunk.
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