InstCombine: Try harder to combine icmp instructions

consider: (and (icmp X, Y), (and Z, (icmp A, B)))
It may be possible to combine (icmp X, Y) with (icmp A, B).
If we successfully combine, create an 'and' instruction with Z.

This fixes PR20814.

N.B. There is room for improvement after this change but I'm not
convinced it's worth chasing yet.

llvm-svn: 216814