When doing the very-late shift-and address-mode optimization,

create a new DAG node to represent the new shift to keep the
DAG consistent, even though it'll almost always be folded into
the address.

If a user of the resulting address has multiple uses, the
nodes may get revisited by a later MatchAddress call, in which
case DAG inconsistencies do matter.

This fixes PR2849.

llvm-svn: 57465