Don't add <imp-def> operands during register rewriting.

Don't add <imp-def> operands during register rewriting.

LiveIntervals already adds <imp-def> operands for super-registers when a subreg
def defines the whole register. Thus, it is not necessary to do it again when
rewriting.

In fact, the super-register imp-defs caused miscompilations because the late
scheduler couldn't see that the super-register was read.

We still add super-reg <imp-use,kill> operands when rewriting virtuals to
physicals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113299 91177308-0d34-0410-b5e6-96231b3b80d8