[DebugInfo] Prepend deref to existing DebugValueInsts#89254
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Debug values have their own, more powerful, type chain check. AllocStackInst have a deref, so this check would be ignored, and AllocBoxInst doesn't support storing a different type.
Load instructions aren't handled in LLVM's salvageDebugInfo due to how unpredictable LLVM's optimization passes can be. In Swift, we only add loads to debug reconstruction blocks if we know it is correct.
Instead of having to recreate new debug value instructions when a deref operator needs to be prepended (in passes where an object is moved to the stack), it is now possible to prepend a deref in place. When a reconstruction block exists, the only correct way to prepend a deref is by adding a load at the start of the reconstruction block, as the reconstruction block is interpreted before the DIExpr. Otherwise, a bit is set on the DebugValueInst to signal that a deref operator must be prepended to the DIExpr returned by getVarInfo. This bit cannot be set if a reconstruction block exists.
Replace uses of createDebugValueAddr in passes, that were used to rewrite debug value instructions with a prepended deref, to use the prependDeref function to prepend it in place.
Replace the use of createDebugValueAddr in salvageLoadDebugInfo, that was used to rewrite debug value instructions with a prepended deref, to use the prependDeref function to prepend it in place, and move it to the correct place.
Remove the now unused SILBuilder::createDebugValueAddr function. Passes must always update existing debug value instructions rather than recreating new ones, and emitDebugDescription has been updated to just prepend an op_deref. This was the last remnant of DebugValueAddrInst.
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Based on #89224
Instead of having to recreate new debug value instructions when a deref operator needs to be prepended (in passes where an object is moved to the stack), it is now possible to prepend a deref in place.
When a reconstruction block exists, the only correct way to prepend a deref is by adding a load at the start of the reconstruction block, as the reconstruction block is interpreted before the DIExpr.
Otherwise, a bit is set on the DebugValueInst to signal that a deref operator must be prepended to the DIExpr returned by getVarInfo.
All passes have been updated to use this new prependDeref rather than SILBuilder::createDebugValueAddr, so this function has also been removed.