[SimplifyCFG] Don't mistake icmp of and for a tree of comparisons

SimplifyCFG tries to turn complex branch conditions into a switch.
Some of it's logic attempts to reason about bitwise arithmetic produced
by InstCombine.  InstCombine can turn things like (X == 2) || (X == 3)
into (X & 1) == 2 and so SimplifyCFG tries to detect when this occurs so
that it can produce a switch instruction.

However, the legality checking was not sufficient to determine whether
or not this had occured.  Correctly check this case by requiring that
the right-hand side of the comparison be a power of two.

This fixes PR26323.

llvm-svn: 258904