[ValueTracking] fix bug computing isKnownToBeAPowerOfTwo() with arithmetic shift right (PR25900)

This is a fix for:
https://llvm.org/bugs/show_bug.cgi?id=25900

If we think that an arithmetic right shift of a power of two is always a power of two, 
an sdiv gets wrongly converted to udiv.

Differential Revision: http://reviews.llvm.org/D15827

llvm-svn: 256655