[SROA] Also insert a bit piece expression if only one piece is needed

Summary: If SROA creates only one piece (e.g. because the other is not needed),
it still needs to create a bit_piece expression if that bit piece is smaller
than the original size of the alloca.

Reviewers: aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16187

llvm-svn: 257795