[PATCH] [BasicAA] Make better use of zext and sign information.

[Nick White]

Hi @hfinkel - is this change OK? `ComputeSignBit` is safe to use in loops as it takes into account phi nodes, and the ` == EK_ZeroEx` check is safe in loops as, no matter how the variable changes between iterations, zero-extensions will always guarantee a zero sign bit. The `isValueEqualInPotentialCycles` check is therefore definitely not needed as all the variable analysis holds no matter how the variables change between loop iterations. Thanks -

Nick

http://reviews.llvm.org/D5866