[LV] Fix crash when target instruction for sinking is dead.

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

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D104603