[DebugInfo][LiveDebugValues] Teach Live Debug Values About Meta Instructions

Previously LiveDebugValues pass would consider meta instructions that 'fiddle' with liveness of registers as register definitions when transfering register defs. This would mean that, for example, a KILL instruction would cause LiveDebugValues to terminate the range of an earlier DBG_VALUE instruction resulting in the none propogation of said DBG_VALUE instructions into later blocks.

This patch adds the check and a helpful comment, fixes a test that previously tested for the broken behaviour by coincidence and adds a test specifically for this.

reviewers: vsk, dstenb, djtodoro

Differential Revision: https://reviews.llvm.org/D73210