Fix Bug 4657: register scavenger asserts with subreg lowering

Fix Bug 4657: register scavenger asserts with subreg lowering

When LowerSubregsInstructionPass::LowerInsert eliminates an INSERT_SUBREG
instriction because it is an identity copy, make sure that the same registers
are alive before and after the elimination.

When the super-register is marked <undef> this requires inserting an
IMPLICIT_DEF instruction to make sure the super register is live.

Fix a related bug where a kill flag on the inserted sub-register was not transferred properly.

Finally, clear the undef flag in MachineInstr::addRegisterKilled. Undef implies dead and kill implies live, so they cant both be valid.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77989 91177308-0d34-0410-b5e6-96231b3b80d8