This is a fix for PR40010 . DBG_VALUE instructions do not contribute to the liveness of virtual registers, making them invisible to the register coaleser, which may merge new register valuations over the top of DBG_VALUEs. When this happens, new valuations can appear to debuggers that were never in the source program, which is misleading.
To avoid this, examine the debug users of the register being eliminated, and pick out any that:
1) Lie in a location where the register isn't live, and
2) Where the destination register _is_ live at that location, and has a different def.
Which means the DBG_VALUE definitely gets resurrected, with a different value. This patch can leave DBG_VALUEs of non-live registers so long as the register isn't written to.
Note this also marks use-before-any-def's as undef, if they're coalesced to a different register, as it's unclear to me how to establish which def a DBG_VALUE is supposed to refer to in that case.