Partially redundant casts

[Daniel Berlin]

Should we bother eliminating partially redundant casts?
I was specifically not handling any unaries in SSAPRE because it was 
way too expensive.
It may not be in the case GVN-PRE that they are.
I'm just curious if it's actually worth it.

Because they are unary expressions, GVN-PRE can detect and eliminate 
partially redundant casts.
But if they don't generate code, this is obviously not worth it.
Is there some way to determine what casts will generate code and what 
won't, so we know what to eliminate?
--Dan