[PATCH] Check for all known bits on ret in InstCombine

[Philip Reames]

The code change LGTM.

The changes inspires two thoughts:
1) As we've discussed off list and Chandler mentioned, we need to 
investigate profitability of various invariant condition propagation 
schemes.
2) When do we remove a llvm.assume?  In your test case, you had an 
assume which (after the transform) retained a condition about an 
otherwise used value (%a).  This seems like an obvious case to remove 
since we've gotten all the benefit we can from it.  Are there other 
cases like this we should implement?

p.s. Do you have a wiki or notes page somewhere where we can track 
potential enhancements?  I worry large parts are getting lost in all the 
various review threads.

Philip

http://reviews.llvm.org/D4567