[DAG] Handle missing transform in fold of value extension case.

Summary:
When pushing an extension of a constant bitwise operator on a load
into the load, change other uses of the load value if they exist to
prevent the old load from persisting.

Reviewers: spatel, RKSimon, efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35030

llvm-svn: 308618