RegCoalescer: Making sure re-materialization defines all subranges

The register coalescer can rematerialize constants that define
more of a register than the copy it is going to replace was going
to do.
This is valid in the case the register was undef before the
copy happened.
This patch makes sure that all the subranges defined by the new
rematerialization instructions have at least a dead def.

Review: http://reviews.llvm.org/D16693
llvm-svn: 259614