[DAG] Avoid Node Update assertion due to AND simplification

Check for construction-time folding for incomplete AND nodes in
BackwardsPropagateMask.

Fixes PR38185.

Reviewers: RKSimon, samparker

Reviewed By: samparker

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D49444

llvm-svn: 337563