Make the peephole optimizer clear kill flags on a vreg if it's about to add new

Make the peephole optimizer clear kill flags on a vreg if it's about to add new
uses of the vreg, since the old kills may no longer be valid.  This was causing
-verify-machineinstrs to complain about uses after kills, and could potentially
have been causing subtle register allocation issues, but I haven't come across a
test case yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151425 91177308-0d34-0410-b5e6-96231b3b80d8