19b65a9c02
In instcombine, when we sink an instruction into a successor block, we try to clone and salvage all the variable assignments that use that Value. This is a behaviour that's (IMO) flawed, but there are important use cases where we want to avoid regressions, thus we're implementing this for the non-instruction debug-info representation. This patch refactors the dbg.value sinking code into it's own function, and installs a parallel implementation for DPValues, the non-instruction debug-info container. This is mostly identical to the dbg.value implementation, except that we don't have an easy-to-access ordering between DPValues, and have to jump through extra hoops to establish one in the (rare) cases where that ordering is required. The test added represents a common use-case in LLVM where these behaviours are important: a loop has been completely optimised away, leaving several dbg.values in a row referring to an instruction that's going to sink. The dbg.values should sink in both dbg.value and RemoveDIs mode, and additionally only the last assignment should sink.
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