This patch implements optimization as mentioned in PR19753: Optimize comparisons with "ashr/lshr exact" of a constanst.
It handles the errors which were seen in PR19958 where wrong code was being emitted due to earlier patch.
Added code for lshr as well as non-exact right shifts.

It implements :
(icmp eq/ne (ashr/lshr const2, A), const1)" ->
(icmp eq/ne A, Log2(const2/const1)) ->
(icmp eq/ne A, Log2(const2) - Log2(const1))

Differential Revision: http://reviews.llvm.org/D4068