[LiveDebugValues] Do not insert DBG_VALUEs after a MBB terminator

This fixes a miscompile that happened because a DBG_VALUE interfered
with the MachineOutliner's liveness analysis.

Inserting a DBG_VALUE after a terminator breaks predicates on MBB such
as isReturnBlock(). And the resulting DBG_VALUE cannot be "live".

I plan to introduce a MachineVerifier check for this situation in a
follow up.

rdar://59859175

Testing: check-llvm, LNT build with a stage2 compiler & entry values
enabled

Differential Revision: https://reviews.llvm.org/D75548