[DebugInfo][SelectionDAG] Do not drop dbg intrinsics with empty metadata locs

Without this patch SelectionDAG silently drops dbg.values using `!{}` operands.

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

This causes assignment-tracking to behaviour to match non-assignment-tracking
behaviour after a recent change (see D140990).

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D150767