Properly handle reloading and spilling around partial redefines in

Properly handle reloading and spilling around partial redefines in
LocalRewriter.

This is a bit of a hack that adds an implicit use operand to model the
read-modify-write nature of a partial redef. Uses and defs are rewritten in
separate passes, and a single operand would never be processed twice.

<rdar://problem/8518892>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116210 91177308-0d34-0410-b5e6-96231b3b80d8