Tighten heuristic for coroutine debug info workaround.

The OutermostLoad condition is supposed to strip the outermost
DW_OP_deref operation because dbg.declares are implicitly
indirect. This patch makes sure the heuristic is only applied to
dbg.declare intrinsics and only if the outermost instruction is a
load.

This was found while qualifying the latest Swift compiler rebranch.

rdar://82037764