Merging r168837: into the 3.2 release branch.

Avoid rewriting instructions twice.

This could cause miscompilations in targets where sub-register
composition is not always idempotent (ARM).

<rdar://problem/12758887>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_32@168849 91177308-0d34-0410-b5e6-96231b3b80d8