[InstCombine] Simplify cttz/ctlz + icmp eq/ne into mask check

Checking whether a number has a certain number of trailing / leading
zeros means checking whether it is of the form XXXX1000 / 0001XXXX,
which can be done with an and+icmp.

Related to https://bugs.llvm.org/show_bug.cgi?id=28668. As a next
step, this can be extended to non-equality predicates.

Differential Revision: https://reviews.llvm.org/D55745

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349530 91177308-0d34-0410-b5e6-96231b3b80d8