[DebugInfo] Avoid dropping debug reconstruction blocks#89412
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This function drops debug_value operands by making them undef, rather than deleting them.
The CapturePromotion and MovedAsyncVarDebugInfoPropagator passes clone debug values without going through the SILCloner, so they don't handle debug basic blocks. Luckily, they should never see debug basic blocks, as they are called before or instead of any optimizations that might create them. Add an assertion to make sure of this.
This new function reverses prependDeref. Mem2reg will call it to remove the deref from a debug_value. If the debug_value relies on the address rather than the value behind it, it kills the operand.
When a store was being salvaged, the reconstruction block was dropped, which could cause invalid debug values to be created. Reconstruction blocks are now cloned with the debug values, and the uses of the address removed by stripDeref. Assisted-by: Claude
If a debug_value on an alloc_stack being SROA'ed has a debug reconstruction block, for example if the variable represents the pointer value directly, its value needs to be dropped and marked as unsalvageable.
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Auditing everywhere DebugValueInst are cloned, changed, or created by passes, to make sure the debug reconstruction blocks remain correct.
PhiExpansion is fixed in a separate PR: #89471
salvage struct&tuple is fixed in #89504