assigned to @lattner

What I don't understand is why the instruction combining pass isn't
eliminating it - it happily eliminates the analogous "and" in various
simpler, artificial, testcases.

The instcombine pass isn't very good at hacking on constant exprs. This should be done by the constant
folding pass.

Implemented, patch here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070319/046236.html

-Chris

Thanks you very much. I feel obliged to point out another possible
simplification though: SExt followed by such an "and" can be turned
into ZExt.

My intention isn't to handle every single case that instcombine does :). If you see a common idiom that a
front-end generates often, we can handle those. Otherwise, they'll get caught by the code generator.

-Chris