The MachineSink pass has a lot of code to handle how variable locations move when an instruction sinks. I wrote in PR44117 how it becomes difficult to determine the correct behaviour in the general case. Dealing with this with instruction-referencing variable locations is simple: we just tolerate DBG_INSTR_REFs referring to values that haven't been computed yet. This patch adds support into InstrRefBasedLDV to record when a variable value appears in the middle of a block, and should have a DBG_VALUE added when it appears (a debug use before def).
While described simply, this relies heavily on the value-propagation algorithm in InstrRefBasedLDV. The implementation doesn't attempt to verify the location of a value unless something non-trivial occurs to merge variable values in vlocJoin. This means that a variable with a value that has no location can retain it across all control flow (including loops). It's only when another debug instruction specifies a different variable value that we have to check, and find there's no location.
This property means that if a machine value is defined in a block dominated by a DBG_INSTR_REF that refers to it, all the successor blocks can automatically find a location for that value (if it's not clobbered). Thus in a sense, InstrRefBasedLDV is already supporting and implementing use-before-defs. This patch allows us to specify a variable location in the block where it's defined.
When loading live-in variable locations, TransferTracker currently discards those where it can't find a location for the variable value. However, we can tell from the machine value number whether the value is defined in this block. If it is, add it to a set of use-before-def records. Then, once the relevant instruction has been processed, emit a DBG_VALUE immediately after it.