[DAGCombiner] Masked merge: enhance handling of 'andn' with immediates

Summary:
Split off from D46031.

The previous patch, D46493, completely disabled unfolding in case of immediates.
But we can do better:
{F6120274} {F6120277}

https://rise4fun.com/Alive/xJS

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: andreadb, llvm-commits

Differential Revision: https://reviews.llvm.org/D46494

llvm-svn: 331685