Fix PR4986. "r1024 = insert_subreg r1024, undef, 2" cannot be turned in an...

Fix PR4986. "r1024 = insert_subreg r1024, undef, 2" cannot be turned in an implicit_def. Instead, it's an identity copy so it should be eliminated. Also make sure to update livevariable kill information.

llvm-svn: 82436