[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