RegAllocFast: Add extra DBG_VALUE for live out spills

This allows LiveDebugValues to insert the proper DBG_VALUEs in live
out blocks if a spill is inserted before the use of a
register. Previously, this would see the register use as the last
DBG_VALUE, even though the stack slot should be treated as the live
out value.

This avoids an lldb test regression when D52010 is re-applied.