InstCombine: Optimize icmp eq/ne (shl Const2, A), Const1

The following implements the optimization for sequences of the form:
icmp eq/ne (shl Const2, A), Const1

Such sequences can be transformed to:
icmp eq/ne A, (TrailingZeros(Const1) - TrailingZeros(Const2))

This handles only the equality operators for now. Other operators need
to be handled.

Patch by Ankur Garg!