ScheduleDAGInstrs: Fix fixupKills()

Rewrite fixupKills() to use the LivePhysRegs class. Simplifies the code
and fixes a bug where the CSR registers in return blocks where missed
leading to invalid kill flags. Also remove the unnecessary rule that we
wouldn't set kill flags on tied operands.

No tests as I have an upcoming commit improving MachineVerifier checks
to catch these cases in multiple existing lit tests.

llvm-svn: 304055