[TwoAddressInstruction] Improve tests for register killed by instruction

[TwoAddressInstruction] Improve tests for register killed by instruction

Define and use a MachineOperand overload of isPlainlyKilled. This
improves codegen in a couple of tests because it catches the case where
MO does not kill Reg but another operand of the same instruction does.

Differential Revision: https://reviews.llvm.org/D147167