[DAG] Fix and-mask folding when narrowing loads.

Summary:
Check that and masks are strictly smaller than implicit mask from
narrowed load.

Fixes PR37820.

Reviewers: samparker, RKSimon, nemanjai

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D48335

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335137 91177308-0d34-0410-b5e6-96231b3b80d8