Allow dead insts to be kept in DeadRemat only when they are rematerializable.

Because isReallyTriviallyReMaterializableGeneric puts many limits on
rematerializable instructions, this fix can prevent instructions with
tied virtual operands and instructions with virtual register uses from
being kept in DeadRemat, so as to workaround the live interval consistency
problem for the dummy instructions kept in DeadRemat.

But we still need to fix the live interval consistency problem. This patch
is just a short time relieve. PR28464 has been filed as a reminder.

Differential Revision: http://reviews.llvm.org/D19486

llvm-svn: 274928