[DebugInfo] Prefer setKillLocation rather than replacing operands with undef

NFC-ish. There is a functional change but the outputs are semantically
identical. Where we might've before replaced one operand with undef (which
means "this is a kill location marker") the use of `setKillLocation` will
replace all location operands with `undef` (which also means "this is a kill
location marker").

Related to https://discourse.llvm.org/t/auto-undef-debug-uses-of-a-deleted-value

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D140904