[DebugInfo] Treat empty metadata operands the same as undef operands

A `ValueAsMetadata` may be replaced with nullptr for several reasons including
deleting values and value remapping a use-before-def. In the case of a
`MetadataAsValue` user, `handleChangedOperand` intercepts and replaces the
metadata with an empty tuple (`!{}`).

At the moment, an empty metadata operand in a debug intrinsics signals that it
can be deleted.

Given that we end up with empty metadata operands in circumstances where the
Value has been "lost" the current behaviour can lead to incorrect variable
locations. Instead, we should treat empty metadata as meaning "there is no
location for the variable" (the same as we currently treat undef operands).

This patch changes `isKillLocation` to take this into account.

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

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D140902