[Assignment Tracking][SROA] Handle DIArgList in migrateDebugInfo

If the to-be-split dbg.assign has a `DIArgList` and a new `Value` has been
requested then use a kill-location for the new dbg.assign. We can't simply
replace the value component (a `DIArgList`) with the new `Value` as that would
leave the `DIExpression` in an invalid state (`DW_OP_LLVM_arg` operands with no
arglist).

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D147312