AArch64: remove all kill flags when extending register liveness.

When we forward a stored value to a load and eliminate it entirely we need to
make sure the liveness of the register is maintained all the way to its use.
Previously we only cleared liveness on the store doing the forwarding, but
there could be other killing uses in between.

We already do the right thing when the load has to be converted into something
else, it was just this one path that skipped it.

llvm-svn: 306318