Handle one offset with growth case seen in povray. Namely, if we have an offset,

Handle one offset with growth case seen in povray.  Namely, if we have an offset,
and the offset lands at a field boundary in the old type, construct a new type,
copying the fields masked by the offset from the old type, and unify with that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26775 91177308-0d34-0410-b5e6-96231b3b80d8