This patch is the counter-part to D67393, and aims to solve the second problem mentioned there, that transfers which are found to be invalid after location propagation occurs should not be inserted. The solution is simple: just don't record transfers until the in-locations are all known to be correct, and then reprocess all instructions a final time creating transfers.
This involves touching every instruction in the program again, which is unfortunate, but an alternate implementation I wrote adds ~150 lines of code to try and unwind chains of transfers, involving a lot of additional complexity. IMO, this is a reasonable trade-off. As a bonus though, because we don't generate a transfer every time we look at a block, we don't generate redundant identical DBG_VALUEs if we re-visit a block.
There's a fractional reduction in scope-bytes-covered with this patch (<0.1%), all which must have been invalid.
(I'll have some asan-build-time measurements of this in a couple of hours).