[LV] Do not try to sink dead instructions.

Dead instructions do not need to be sunk. Currently we try and record
the recipies for them, but there are no recipes emitted for them and
there's nothing to sink. They can be removed from SinkAfter while
marking them for recording.

Fixes PR44634.

Reviewers: rengolin, hsaito, fhahn, Ayal, gilr

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D73423

(cherry picked from commit a911fef3)