This patch fixes a crash when the target instruction for sinking is
dead. In that case, no recipe is created and trying to get the recipe
for it results in a crash. To ensure all sink targets are alive, find &
use the first previous alive instruction.
Note that the case where the sink source is dead is already handled.
Found by
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=35320
It may be better to handle all updates of SinkAfter due to DeadInstructions in one place, i.e., erasing dead sinks and bumping dead targets, either both here or both below inside buildVPlanWithVPRecipes(Range).